Part of the problem is that the area the Bulgars conquered had been Byzantine and the peasants were all Orthodox.
Yes, life got crazy, and the population ended up being replaced by Slavs, but the Orthodox had feet on the ground. The Catholics didn't.
If Boris (and the 'Bulgar' rulers) converted to Catholicism, I suspect that would hurt the legitimacy of the government, and make it easier for the Byzantines to take and hold. Or a different barbarian tribe that was Orthodox would take over... (Note that, in the West, none of the Arian Goth and Vandal states survived, whereas the Catholic Franks did.)