Slate interviews John Pfaff, a professor at Fordham Law School who's analyzed the statistics. It's not really because of drug busts or long sentences. We've got two periods to consider:
You need to break the question into two periods. Because there’s a time between 1975 and 1991 when you see this dramatic rise in crime, and the prison population went up as well. And then there’s a more interesting period, between 1991 and 2010, when crime steadily declined, yet prison populations kept going up. So, between ’75 and ’91, it’s almost certain that the increase in crime had to play at least some significant role in increasing the prison population....What appears to happen during this time—the years I look at are 1994 to 2008, just based on the data that’s available—is that the probability that a district attorneys file a felony charge against an arrestee goes from about 1 in 3, to 2 in 3. So over the course of the ’90s and 2000s, district attorneys just got much more aggressive in how they filed charges. Defendants who they would not have filed felony charges against before, they now are charging with felonies. I can’t tell you why they’re doing that. No one’s really got an answer to that yet. But it does seem that the number of felony cases filed shoots up very strongly, even as the number of arrests goes down.
My pet theory, of course, is that when the Cold War shut down in the early 1990s the country had to do something to take up the slack in anxiety relief. We could no longer get angry as the Soviets. So, we decided to direct our anger at our own citizens and jack up the prison population.
That's MY explanation. I like it and it's consistent with Pfaff's story. That is, he has no story. We just have local DAs filing more felony indictments, but we don't really know why they're doing that.
I note, though, that mere consistency with lack of knowledge is not exactly strong empirical validation.