Because he's effectively immortal - can't be killed. The bigger सवाल for me is, why haven't they tried? Perhaps their thought is that trying to do so will just cause him to evolve once again with the Hougyoku still in his chest, but they've got him pretty effectively suppressed. Seems like it would have been pretty nice to torture him for a while and test the limits of his regeneration, which at the very least causes him some major torment.
Though this is all besides the point. I'm hoping that Aizen comes back into the story, which may be the reason Kubo's kept him around. We'll see.