Was it on the basis of the devastation of the flood that man was allowed animal flesh?

No plants were brought on the ark.

And nothing was revealed how they repopulated the earth again post flood.

If we are to guess a reason for the diet change post flood, we can perhaps say not all the pre flood plants made it post flood, and thus a vegetarian diet was no longer adequate for man, as God sees it.

A trivial example is today's diet is lentils which is incomplete unless combined with rice for the full supplement of amino acids required by man.

Or consider the carnivores. Pre flood they too were vegetarian, and we all know cats cannot survive without meat today.

So apparently something fundamentally changed in the plants on earth post flood, for which God now allow meat to be part of man and the creatures' diet. But we must confess ignorance to what this change was. All we know is the new provision by God.

Was it on the basis of the devastation of the flood that man was allowed animal flesh?

I think not. The reason, I believe, is found from the wickedness of the earth in the time before the flood.

After God created man, He gave them authority over the fish in the sea, and the birds in the sky and over every living creature that moves on the ground.

From Genesis 1:

28 God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish in the sea and the birds in the sky and over every living creature that moves on the ground.”

God then told Adam and woman:

29 Then God said, “I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food.

And this extended to all of the air breathing creatures:

30 And to all the beasts of the earth and all the birds in the sky and all the creatures that move along the ground—everything that has the breath of life in it—I give every green plant for food.” And it was so.

In the beginning, nothing ate meat!

I'll start with Genesis 4.

The story of Cain and Able is interesting. Cain worked the soil, and Able tended his flocks. Soil is described throughout Scripture as something that man 'has that he can develop', and 'tending flocks' describes a relationship of one who actively works to ensure the health and welfare of those he cares for. Genesis 4 states:

2 . . .Now Abel kept flocks, and Cain worked the soil. 3 In the course of time Cain brought some of the fruits of the soil as an offering to the Lord. 4 And Abel also brought an offering—fat portions from some of the firstborn of his flock. The Lord looked with favor on Abel and his offering, 5 but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.

As God was not pleased with Cains offering, "working the soil" must describe how Cain interacted with the lives of those whom he had control over. Beating others to force compliance, lying, cheating, stealing, and killing, all describe a few harsh methods of "working the soil". The results of Cains harsh and ugly treatment of those he had control over would have been his offering to God.
Able, on the other hand, tended flocks. Tending flocks indicates a relationship of one who nurtures and cares for others. And because Able's offering pleased God, Ables relationship with his family and others would have been a loving one.

Psalm 23 states:

1 The Lord is my shepherd, I lack nothing. 2 He makes me lie down in green pastures, he leads me beside quiet waters, 3 he refreshes my soul. He guides me along the right paths for his name’s sake. 4 Even though I walk through the darkest valley, I will fear no evil, for you are with me; your rod and your staff, they comfort me.

Able was a shepherd of people, and Cain was a user of people. Cain was resentful that God was pleased with his brother and not him, so he killed Able and felt no remorse.

Perhaps God blessed Able, and not Cain, with prosperity. And perhaps Cain resented his brother because of his wealth.

The people at that time would have been genetically perfect. Disease would have been non-existent. People at that time lived incredibly long lives and the females would have had many, many children. All females born before the flood would have been taken by any man at any time unless they were protected by a strong family.

It appears from the verses of Genesis 3 that man could speak with the animals. As man had direct control over the animals, the animals would do mans bidding. Over the course of the years before the flood, even the animals became wicked.

Genesis 6 states:

5 The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. 6 The Lord regretted that he had made human beings on the earth, and his heart was deeply troubled. 7 So the Lord said, “I will wipe from the face of the earth the human race I have created—and with them the animals, the birds and the creatures that move along the ground—for I regret that I have made them. 8 But Noah found favor in the eyes of the Lord.”

By the time of the flood of Noah, man and the animals had progressed from eating from the seed-bearing plants on the face of the earth and every tree that had fruit with seed in it, and every green plant, to eating meat. Even Noah and his family ate meat.

Was it on the basis that after the devastation of the flood, man was allowed animal flesh?

1And God blessed Noah and his sons, and said unto them, Be fruitful, and multiply, and replenish the earth. 2And the fear of you and the dread of you shall be upon every beast of the earth, and upon every fowl of the air, upon all that moveth upon the earth, and upon all the fishes of the sea; into your hand are they delivered. 3Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things. 4But flesh with the life thereof, which is the blood thereof, shall ye not eat. 5And surely your blood of your lives will I require; at the hand of every beast will I require it, and at the hand of man; at the hand of every man's brother will I require the life of man. - Genesis 9: 1-5

Sacred Scriptures give us the biggest clue to this subject matter.

Meat After the Flood

There are several answers to this offered by the commentators. As you will see, most are basically practical – that in spite of Nachmanides’ objections, postdiluvian man had a greater need for meat or stronger rights over the animal kingdom than his predecessors. There are, however, approaches along the lines that a more fundamental change occurred in the relationship between mankind and the animal kingdom after the Flood.

(1) The animals survived the flood only because of Noah’s efforts. Since they owed their lives to him, Noah and his descendants had gained rights over them – including the right to consume them (Ramban (1:29), R. Bechaye, Bechor Shor, Chizkuni, Ohr HaChaim, Malbim). Further, Noah and his family exerted great effort to keep the animals alive on the ark. He also offered sacrifices to God after the Flood – evoking a guarantee from God that He would never again destroy the Earth and its animal life. Thus, Noah and his descendants earned the right to use animals for their needs (Ohr HaChaim).

(2) Man was weaker after the Flood (as reflected in his shorter lifespan). The world’s flora was also of inferior quality to that which existed before the Flood. The world would now have seasons, with long unproductive winters. Man was also destined to spread out further – to colder areas of the globe. As a result, he would require meat to survive (Malbim, R. Hirsch). Man’s shorter lifespan may have also indicated that his life would be more hectic and he would require more nourishment (R. Hirsch).

(3) Noah and his family came out of the ark to a barren world, with nothing to subsist on. They were allowed to eat meat in order to survive – and that became permissible for all time (Abarbanel).

(4) Man reached a very low level of morality before the Flood, reducing human beings to the level of animals. People paid little heed to their souls, created in the Divine image, and basically began acting like animals. (This is why Cain offered plants in sacrifice to God rather than animals (Gen. 4:3). He saw animals as the equal of man. We could not kill them even in God’s service. Once the distinction between animal and man became blurred, Cain’s next step (once he realized animal life could at times be taken – as Abel did) was to kill a human.) God therefore saw need to permit animals to man – to emphasize the fundamental difference between the animal soul and the human one. Man would hopefully then begin to appreciate his humanity (Sefer Ha’Ikkarim). (For a more detailed treatment of this approach, see here and here.)

(5) With the restart of the human race with Noah and his family, man would reach higher levels of spirituality than before – culminating in the Jewish people. The distinction between man and animal would become much greater. Once this became the case, eating animals in fact became desirable. Spiritually speaking, when a higher form of life consumes a lower one, it serves to elevate the lower one, enabling it to serve and become a part of something greater than itself. (See Malbim and Kli Yakar.) The Talmud likewise states that only a Torah scholar should eat meat, not an ignoramus (Pesachim 49b). In other words, only when the consumption will truly be an elevation for the meat is such behavior appropriate.

(6) It’s possible to suggest that the spiritual level of the animals declined somewhat with the world’s destruction and recreation at the time of the Flood. Before the Flood, the physical world was a much more spiritually charged place. The bonds linking the heavens to the earth were much more powerful. The spiritual vitality of the upper spheres energized the earth to a much greater extent. The earth was thus a richer, more verdant place, and human beings lived much longer.

Just before the flood came upon the earth, God told Noah that man’s lifespan would be 120 years. To me, this really suggests that man will have a weaker physical constitution after the flood and thus God permitted man to eat the flesh of living animals.

3 And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. - Genesis 6:3