>I assumed plea bargains were always this popular. Any idea why they rose in popularity during this specific period?

A variety of reasons but mandatory minimums and stiffer sentencing in general (both made popular by the war on drugs) are the bulk of the reason.

When your lawyer could say in confidence that based on the fact pattern you're all but assured sentence on the order of a lesser crime that more correctly fits the fact pattern you're more likely to go to court and try and clear your name. When you're guaranteed a minimum sentence should you lose taking the bargain seems more reasonable.