They aren't core, because they, well, aren't. WotC decides what it's core, and that's the three books and no more.
I don't think they should be core, either. Psionics aren't too much entrenched in fantasy tradition. Now if they decided to file the serials off and call it a magic system, it might have a core place - that is, if slot-based magic wasn't a sacred cow. Besides, I like slot-based magic. Mana points feel like a videogame.
I like a minimal cross-referencing. Enough that noncore books have a bit of support, but not so much that you need those noncore books.