While we may not have pushed for zero tolerance in general if a politician comes out and says he thinks we are over charging people or giving non-violent offenders lesser sentences their opponent will pound them for being "soft on crime" and it will work. Hell, we elect prosecutors and even judges in most places. How does a prosecutor get reelected, he tells you his "98% conviction rate" which is an irrelevant number. The relevant number is how many guilty people he convicted. They have a major incentive at throwing your ass in jail, guilt be damned.
Then there are the judges who must campaign. The one person who is supposed to be completely impartial is sitting in a courtroom deciding cases with lawyers who have donated to his campaign and you better believe the smart ones donated. How can we possibly expect a judge to solicit funds to run a campaign and remain completely impartial when one of the lawyers backed him in the election and the other guy didn't or even worse backed his opponent. How can people possibly ascertain which guy has a better understanding of the law from 30 second campaign ads? This is another one that we elect "hard on crime" judges, often boasting of how many hours they sentenced people in total in their 30 second ads. I read an article, I'll try to find it, that there is a direct correlation between how long your ass is going to get thrown in jail and the judges election cycle. Basically, you really really don't want to get sentenced during an election year and if you do you better make damn sure your lawyer has donated the max to the judge.