User blog comment:Espada2011/Aizen's power/@comment-11051098-20140404183121/@comment-24306779-20140405093409

The Hogyoku abandoned Aizen most likely because Aizen's resolve to become a god was weakened by the discovery that someone as strong as him really existed; That's what Ichigo felt by crossing swords with him. The Hogyoku helps his user to fulfill his desires; if the user's desire lessen so does the Hogyoku's effect.

Anyway, Aizen had so much Reiryoku at that point that he probably didn't need a zanpakuto to influence his surroundings.