San Francisco is not meaningfully a one-party city. There are sub-party factions (the "progressives" and "moderates") who vote as blocs on the most significant issues and tend to get similar groups of endorsements. The Republicans have been run out of town (for good reason), so now the Democrats, which are a big-tent party, can - and do - safely fight amongst themselves. And it's not just primaries, either; ranked-choice voting means that general elections for city positions are usually contested between two (or more) Democrats even in November.