If God is all mighty,why doesn't he change bad people/seiners into good ones?

[Ryantology]

It works pretty well in practice, too. If you don't trust the person not to cheat or become a junkie, you probably shouldn't merry them in the first place.

They could be a saint, now, but that doesn't mean their past activities don't carry possible present consequences.

Catholic doctrine would work perfectly in a world where sexual drives didn't exist and marriages were entirely open affairs between two equal, rational and careful people done only out of mutual desire and love. In the real world, Catholic doctrine kills thousands by making a bad problem unnecessarily worse.

Have they prevented the sale of condoms in any nations?

If they had the ability to do that, do you doubt they would?

Instead, they simply scare poor, hungry and uneducated people with superstitious babble into effectively achieving the next best result. And, thousands and thousands die because of it. To the Church, it is better that they die in protracted agony than that they protect themselves in 'sin'.