It's difficult to support other settings because wotc isn't willing to make mechanics changes or strip away FR lore & baselines to make room for other settings . They were only willing to do it for eberron after a dmsguild release stripping away FR to rebuild the races as thy fit the setting justified the creation of the adamantine dmsguild medal here & a second one doing similar for classes was mithral.To think we could've had 1000s more subclasses by now!
Not to mention all the existing settings that could've been repackaged and released in order to validate fans preferences for those settings.