There are many factors. The biggest is the failed drug war. Conservatives thought that by getting tough on crime and drugs that those problems could be eliminated. The result was the most full prisons on the planet. The prisons are so full that even the huge California GDP can barely pay for them. So something had to ease up. Now there is a breakdown in the justice system regarding what crimes are worth convicting and incarcerating people for and it is not unusual for even violent criminals to be quickly released.

But the conservative tough on crime folks are completely unable to come to terms with the consequences of their actions on society. So we end up with a stalemate where liberals are trying to trim down the prison population and focus on violent crime while most conservatives still want to fix the problem by getting tough and imprisoning more people for longer.