I think the impediment to WHR is primarily psychological. Players who don’t pay close attention to rating systems (most of them) generally don’t like it when their rank changes when they aren’t playing. Even if you only update ratings when someone plays a game, many people complain loudly when their rank goes down after winning a game. (OGS currently uses a history window, so this can sometimes happen. You can find many complaints in the message boards.) I think WHR would likely increase the frequency of this issue, and the comparative gain in rating accuracy might not be worth it.

I think one solution that would allow the use of a more accurate rating system would be to disconnect rank from rating, as GoQuest does. Having your rank reflect your maximum achievement while your rating reflects your current level softens the blow of losing a few rating points. There’s a lot of history in using ranks to decide matchups and handicaps, though, so that idea might meet resistance in the community.