Because you have a major war with Luria Nova coming up and having Astroist priests kicking your lords out of their regions could be bad for business?
Ooooh, I'd love to see Luria Nova use priests against a faithful realm, after declaring war on a theocracy. How very pious of them.
Besides, most of our nobles are astroists anyways, and Lurians can't claim regions from astroist lords. And both D'Hara and Morek have their own astroist priests... that could go both ways.
If it comes down to this, however, it'll finally clearly be made clear why not being able to kick priests out of a religion is a stupid mechanic.