Why would they? if WoTC defines magic in any coherent and systematic way they are going to alienate any and all that disagree on their interpretation, and they will not buy their game.Oh, I've got this one:
Because WotC never spent any time considering what 'magic' means to any of their settings and as a result now neither they nor a vast portion of the fandom really have any frame of reference for how it should, could or would work aside from pulling a Quesada and saying 'it's magic, we don't have to explain it'.
Look at all the angst and drama over lore and changes to lore in revisions of older settings here and elsewhere. If magic systematized that become an iron band and no deviation will be allowed by sections of the fanbase. They are never going to bind the hand of new DMs and designers in that fashion. It is bad for business and creativity.