I would point to the massive expansion of government regulation starting in the early progressive era, expanding even more in the 1930's and really exploding in the 1960's, expanding even more with the "War on Drugs" and "War on Terrorism". As it's turned out, while the initial roots were on the political left, the political right has embraced it as well. Bludgeoning ordinary citizens with the law is something both left and right wing mainstream US politicians can agree on.
Starting in the mid-1960's the incarceration rate started increasing and really took off by the mid-1980's.
Judges were, by law, given less discretion in handing out sentences. This came about for two reasons. First, some sought fair application of the law. They didn't want to see one person getting off with probation while another got 10 years, particularly when this difference seemed to be based on race. Other politicians didn't want to be seen as being "soft on crime" or wanted to be seen as "tough on crime". Therefore, mandatory sentencing laws were passed that killed a lot of judicial discretion that was the case previously.
Then, instead of cutting back on regulation, politicians chose to criminalize just about everything (see 3 Felonies a Day to see how this works). This allowed for harassment of political opponents, stifling of dissent and generally controlling the population. This, coupled with the curtailment of judicial discretion has led to the situation we have today, where intent doesn't matter as much.