It didn't "take Trump".
Very few people outside of the Black community and hard-left liberals were talking about prison reform in 2009. Obama DID push though a partial correction of the crack cocaine sentencing disparity and pardoned a record number of people, but the major reform movement wasn't there when he came into office and it had no chance of passing among moderate dems or republicans.
The prison reform wave didn't go mainstream until around 2012. At that point Holder and Obama both pushed it and much stronger prison reform and sentencing bills were put forth by Democrats repeatedly. But Republican leadership refused to sign.
When Trump came into office, Democrats continued to put forth serious prison reform legislation with some libertarian (Rand Paul) and a little bit of other Republican support. But AG Jeff Sessions was one of the hardest anti-reform a$$hole AG's in history and Trump doesn't actually give a shyt about that stuff (remember the Central Park 5?) so it didn't go anywhere at first. But as the movement grew, some Republicans signaled that they would support a much weaker version of the bill. Trump's son-in-law was a big supporter of the cause and in his ear, so Trump finally said that he'd support the weaker bill, and so Republican leadership let it through.
That's how it went down. That weak-ass reform bill would have happened under ANY president at this point in history. The problem was that the centrist support for even that didn't come around until the last five years.