probably because there are more fixed limit players in general.

I am a NL player, and strongly recommend heads-up NL play. In a ring game, you can usually tell within a couple minutes wether you'll stack the guy you're playing heads-up.

I am totally biased (I hate fixed limit poker in general), but I think that heads-up limit games are silly, unless it's in sit n go form, where you're guaranteed to see enough of your opponent's hands to get a good feel for him.