Good question!
1. It is wholly unpopular with parole boards to say things like, "I'm an atheist! Morality is subjective and the laws of this state, a societal contrivance."
2. Is is POPULAR to claim Christian conversion in interviews, polls, parole boards to secure early release if possible--because the authorities see the 180-degree turn born agains make in prison.
3. You seem to be drawing an inference that Christianity gets people jailed while atheism has lawful citizens. I do not advertise Christianity as "spend less time in prison!" as if that's something laudable, just as atheists seem to say they are good despite the fact that policemen don't pull people over for obeying the law. "Hi, sorry to stop your vehicle, but you were going 68 in a 70, and I feel that I owe you a debt of gratitude for being safe." This doesn't play in Heaven, either--"Let me in, God, because although I was an atheist, I did a bunch of good things for self-preservation and to avoid jail time." So atheists are on the whole better educated and wealthier than other sinners and have better lawyers . . . that has to account in part for the statistics, because it is so--atheists tend to have more college degrees and more money than other sinners.
4. Again, a lot of people come to Jesus for salvation when the chips are down. Jesus said it was a shame that prostitutes and tax collectors were crowding the entrance door of Heaven while the religious were missing the door completely. I EXPECT to see MANY Christians in prison. On death row, a lot of people either get saved or claim to have gotten saved. They are facing an existential crisis as best they can--you don't really ever here about serial killers embracing atheism afresh in prison--what I hear is that atheists dislike talking about the grave and what comes after.