Why should contraceptives not be cheap and widely available?

God commanded humans to "be fruitful and multiply" (Genesis 1:28). He also gave all sorts of commands regarding marriage and sexual morality. In summary, man is made for woman and woman for man, and the purpose of this Holy Union is to "be fruitful and multiply". Hence, not only adultery, but also h*m*sexuality and transgenderism are excluded. Children are the intended outcome of this union, and in particular, are the intended result of sexual intercourse.

Now, it's true the abortion is homicide. (In most, but not all, cases, we can say further that abortion is murder, but there are exceptions.) However, that is not the whole story. Many Christians see the division as not (only) between those who would murder defenseless children, but between those in obedience to God's commands, and those who are disobeying Genesis 1:28. From this perspective, pro-abortion attitudes are a natural extension of people that demand sexual intercourse outside of God's plan. That these same people define pregnancy as a "consequence", rather than the blessing God intended, shows rather starkly the perversion of this thinking. (And, of course, contraception makes it much easier to engage in adultery without "complications".)

In other words, many Christians view the issue as not merely about abortion, but about those who obey God versus those who hate children and will do anything to avoid having them, in contradiction to God's command. Note that the argument is not that contraception is homicide (though I wouldn't be surprised if some do make that argument), but that the question of abortion is a subset of the real issue wherein abortion and contraception both arise from the same underlying cause.

To your question then, these Christians would see little legitimate use for contraceptives; at least (see also below) no legitimate reason for them to be widely available. What benefit is there to something which enables sin being cheap and widely available?

As an aside, "contraception" is the deliberate prevention of conception. Other users have noted that there may be legitimate uses of "contraceptives" when the primary purpose of use is not contraception. The prevention of implantation, however, would be considered murder by most Christians.

Then God blessed them, and God said to them, “Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth.”
— Genesis 1:28

It all boils down to how each denomination interprets the Hebrew word מָלָא, which is often translated as "replenish" or "fill".

Some believe that "be fruitful and multiply" means to continue increasing the population until the end of the age.
For them, the only purpose of contraceptives is to avoid obeying that commandment, and so should be banned.

Others believe that we have already replenished and subdued the Earth; that for most of history our population has been in the hundreds of millions and sat comfortably under a billion until relatively recently (about 200 years ago); and that the Earth can't support its current load of nearly ten times that number (“The earth lies polluted under its inhabitants”).
For them, overpopulation is the obvious cause of the current environmental disasters, and in light of modern medicine that now allows most children to survive to have their own families, contraceptives and deliberate family planning are the obvious solution to this problem.
(Consider that 90% of all pollution wouldn't exist had we kept our population at around one billion.)