At local and state levels the (R) still outnumbers the (D) in most states. Americans tend to vote progressive at the national level, and stay more conservative at the local level.
Doing this would clear the wrong socioeconomics groups of past offenses for that party. As well as remove the heavy economic fines they pay as previously convicted felons. It would open the door to a large contingent of that socioeconomic group requesting voting rights back as well, since vacated or cleared charges means restoration of rights.
In summary, there is no impetus for the dominant party in most local politics to do this. It dilutes there voting bloc, removes a questionable but lucrative revenue stream, and de-establishes a power structure used to control a portion of the population. All the reasons the Nixon White House came up with the idea of criminalizing drugs in the first place.
Highly relevant quote;
A top Nixon aide, John Ehrlichman, later admitted: “You want to know what this was really all about. The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar left and black people. You understand what I’m saying. We knew we couldn’t make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did.”