Perhaps the fact that it's still pretty new? (I mean, they're still trying to work out the PACL issues...)
Personally I think it may mostly be due to who the users are. Most people do not download, install, and hack on Liferay for the fun of it. It's mostly corporate developers, ones who either bring it in house on their own or from a mandate on high. In either case, the enterprise typically has a narrow set of goals (create a liferay theme that matches our current web page theme, get Liferay integrated into our own SSO, etc.), ones that are either not generic enough to be useful for others, they may expose too much of the corp's systems/datastructures/other to be seen as 'safe to release'.
My bosses give me a hard enough time when I say I want to use some piece of open source in house ("is it GPL?" "Remember we can't use that one type of license that's bad for business; I don't remember the name of it, but I just know we can't use it..."), let alone if I said I wanted to release something we've done (the push now is to get patents on everything - new way to wipe your own a$$, let's patent it before someone else does).
Personally I do some side things in Liferay when I get the time, but that tends to be the rub. What few cycles I get now I give to the forum, but I would like to get some more time to get some plugins out there...
