Greg will of course correct me if I'm wrong - but I suspect the problem is that lots of people want CC-by-sa because it's easier to reuse stuff ... but that GFDL makes it hard to reuse stuff is considered a *feature* by many, e.g. photographers who license work as GFDL but also sell it privately. That is: the thing that makes GFDL a pain in the backside for a wiki is precisely why they like it, and they want it to stay a pain in the backside for that reason.
Anybody who values this "feature" for use with WMF projects probably doesn't understand what exactly we are all trying to do here.
--Andrew Whitworth