He never said he would legalize marijuana. He can’t do that, wrong branch. He did say he would pursue decriminalization of it, which he did pursue Iike the other comment said about trying to get it rescheduled. As of January 2022 there were 0 people in federal prisons for charges solely based upon possession of marijuana. (He pardoned a number to get them out)

If you want it legalized, look at the legislative branch.