This is widely believed but turns out to be false. The average length of prison stays may not have increased significantly at all in the last 50 years. Increased prison sentences are not the cause of mass incarceration.

What do you believe is the biggest factor that contributes to mass incarceration?

The best thing I've read on this, which is the Pfaff study on mass incarceration and the drug war, suggests that the biggest issue is that even as crime dropped steadily during the mid-late 1990s, prosecutors offices continued staffing up, and prosecutors themselves became more likely to file severe charges.

So the short answer is "prosecutorial discretion".

The longer answer might be something like this:

During the crime wave from the late 70s through the early 90s, the American public demanded increasingly aggressive prosecution of crime. Prosecutors offices staffed up in response to it. People were elected on promises of increasing funding both to police and to prosecutors. Prosecutors themselves were incentivized to file more cases, again due to the public mood.

The crime wave passed, but once set in motion, the systemic increase in prosecutions didn't stop: in large organizations, headcount, once obtained, is only released dearly. Managed business objectives (like expectations on percentage of offenders for whom prosecutors will charge with felonies), once established, are only with great effort ever changed.

And so that's where we're at now: a prosecutorial system calibrated to the crime levels and hysterical public mood of the late 1980s.

Very interesting. I have been paying attention, but I guess not well enough. That makes sense and falls along the same line of thinking as... you make what you measure.