
Doom 3, Quake 4 and ETQW all had official SDK's and probably more documentation released initially than even Quake 3 (most of that documentation was built up by years worth of community members). While I agree that SDK's help provide a game with a bigger community, it's not the primary factor.
Both Doom 3 and Quake 4 sold well, but didn't develop much of a community following (thus you not hearing very much buzz about it post release) due to what I think is a lack of decent multiplayer. Single player games tend to promote more "closeted" players who will play through the single player campaigns once or twice and while they may really enjoy it, there is little need to go online and start up a community to talk about it.
Multiplayer games are by nature social, so you are more likely to have players on an online forum talking about the game, planning strategies, setting up matches, etc. This type of community is more likely to spawn a group of people willing to create custom content, so that they actually have a crowd of players to tailor to.