^He talked about it non stop^
It was literally one of the pillars of his platform. Aside from his Medicare for all bit, I can't think of something that he harped on more than campaign finance reform and repealing CU.
Hillary on the other hand....one could actually make an argument that she is responsible for the Citizens United ruling in the first place! A non-profit group made a documentary film during the run-up to the 2008 election called "Hillary: The Movie", that was not very flattering towards her, and she complained to the FEC, and in turn they blocked its release stating that it violated some obscure election rule, and presto! Citizens United was born, which went all the way to the Supreme Court, and we now have that very ruling perverting our elections.