Why did people live so long before the Flood?

A very interesting question. I believe the answer is very simple.

The lifespan of man was very high before the flood because Man had before sin partaken of The Tree of Life.

The Bible says of the tree of life:

Genesis 3:22

And the LORD God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live for ever:

People who eat of this tree will have enormously long life. If one would continue to eat from the Tree of Life then (s)he would live forever. This tree was specifically designed to prolong the life of man.

Once we reach heaven man is once again permitted to eat of the fruit of the tree of life:

Revelation 22:14

Blessed are they that do his commandments, that they may have right to the tree of life, and may enter in through the gates into the city.

Infact even the leaves of the tree of life are for the healing of the nations (see Revelation 22:2).

A few more causes that added to the above fact are:

1. The change in the diet plan after flood: Before flood the divinely appointed food was vegetarian, no meat was allowed, but after flood meat was allowed as a part of food - this is one of the most significant changes that directly affected man. Today we find so many arguements stating the effect of meat eating on health. No one can possibly deny this. Infact God may have allowed this habit to specifically shorten man's lifespan. Longer lifespan begat much sin - so much that God was forced to destroy the earth, so God may have purposefully shortened man's lifespan to limit sin.

2. The change in the climatic condition after flood

The Bible tells us that the Earth was a very different place back in Noah's time and there were several factors which allowed people to live extremely long lifespans.

Back before the flood, it did not even rain in that time:

Genesis 2:5 – “Neither wild plants nor grains were growing on the earth, for the LORD God had not yet sent rain to water the earth and there were no people to cultivate the soil. (6) Instead, springs came up from the ground and watered all the land.”

However, In Genesis 1:6 we are told that God did place vast amounts of water above the sky, however this water could not have been in the form of regular clouds – as it did not rain:

Genesis 1:6 – “And God said, ‘let there be an expanse in the midst of the waters, and let it separate the waters from the waters.’ (7) And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse. And it was so."

The NIV translates 2 Peter 3:5 in this way – “They deliberately forget that God made the heavens by the word of his command, and he brought the earth out from the water and surrounded it with water.”

This trend of no rain happening was still occurring in Noah's time, because when Noah and his family exit the Ark God sais:

Genesis 9:13 – “I shall set my bow in the clouds and it shall be for a sign of a covenant between me and the earth.”

Before that time there would have been no rainbows, because large scale precipitation was not happening and therefore no rain or clouds to cause rainbows in the sky.

Also we are told in Hebrews 11:7 - "But by faith, when Noah was warned about things that had never been seen, he built an Ark to save his family."

Things like rain and storms would not have been seen before in Noah’s time.

Genesis 7:11 – “In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.”

A floodgate or a window is something which typically holds air or water in or out of somewhere. These windows ‘opened’ unleashing the vast reservoirs of water that were suspended above the earth’s atmosphere; which were a major source of the water that caused the flood.

Genesis 7:12 – “And rain fell on the earth forty days and forty nights.”

This is the first mention of rain and it fell in such quantities that it took 40 whole days and nights’ non-stop for it to cease falling.

This layer of water above the sky mentioned in Genesis 1:6 could very well have been suspended in space just outside the earth's atmosphere, where the water would be frozen as ice crystals. The planet’s gravity would be holding it in place, similar to Saturn's rings which are also composed mainly of ice crystals. But this layer would have been a shroud that completely surrounded the earth.

Ice is of course translucent and so the suns light would be able to easily pass through, but at the same time this layer would be filtering much of the harmful radiation coming from space. The light and heat at ground level would probably have been less intense back then, but also the vapour canopy would stop heat escaping into the atmosphere making the planets temperatures far more moderate and less extreme than what they are today.

This water canopy would also condense the air beneath it, increasing the air pressure at ground level. This meant lots more oxygen per square inch for an organism to take in at one time. This is called a hyperbaric condition. It promotes rapid growth and large size as well as rapid healing and cell regeneration.

In October 2006 Science Daily publicized a study led by Arizona State University staff entitled “Giant Insects Might Reign If Only There Was More Oxygen in the Air.”
“Recently, USGS scientists have used a gas QMS to determine the oxygen level of ancient samples of Earth's atmosphere from a most unlikely place - amber. …Minute bubbles of ancient air trapped by successive flows of tree resin during the life of the tree are preserved in the amber. Analyses of the gases in these bubbles show that the earth s atmosphere once contained nearly 35 percent oxygen compared to present levels of 21 percent.”

This combined with the fact that harmful radiation from space which mutates cells was virtually non-existent; animals, plants, people and other organisms could live up to ten times longer than what is possible today!

The "canopy" is an extra-biblical answer, but one generally given by the Institute for Creation Research and Answers In Genesis has to do with the climate pre-flood. During the creation, it was said that the waters above were separated from the waters below, and during the flood, it is said that water came from both places. The implication is that there was a canopy of water that protected human life from cosmic radiation, etc...

I'm not saying this is the answer, just one that is commonly cited.

Because God said so :-)

I can think of several practical reasons but they are mostly speculation. In order to not let the natural process of decay and corruption mess up the gene pool too much he may have let people live longer early on. See: Is incest a sin?

Additionally this would have helped with the minimum number of people needed for a stand-alone society to survive problem. As the world got harder to live in after the fall, this number would have gone up faster than the population growth. With a low death rate, the system got a kind of jump-start.

I could go on, but that's enough speculation for one answer.

What we do know is why he shortened it. In Genesis 6:3 we find God abruptly shortening man's lifespan to 120 years. I think we can think of this about like the SE rep-cap. There is no promise that we will live that long, just that's the upper limit. There are exceptions (bonuses, accepted answer points) that get you a little over the line too, but it's a pretty solid rule of thumb. As for why it happened, apparently man wasn't getting smarter and holier with more time on his hands, only more evil and God didn't think that was healthy for us.