Because FR is the Borg of D&D. Everything gets assimilated. Take a look how the Greyhawk demihuman deities became FR deities, for example.
What I found amazing along the same lines was one of the spells dealing with portals is only available to wizards in MotP is available to all major spellcasting classes in FR. When querried about why this would be, a WotC staffer quipped that people in FR have a more everpresent need for the spell.
To which my reaction was: WHAT!? The planar cosmology is all about portals.
I dunno. I think this is my number one beef with FR... it's just gotta have everything, and as a result, it is an astounding mishmash of setting elements.