For the same reason other states in a similar conundrum didn't do the same. They'd be fighting a 2-front war.

Bulgarian Empire was an expansionist Slavic state to the north of European Byzantine Empire, and the two countries were more or less constantly at war with one another.

Its population was also Bulgarian, spoke a different language, came from a different culture, and they most definitely weren't Greeks. As such, while Byzantium could probably go all-out and conquer Bulgaria, but they would never be able to hold it. It take a significant amount of forces to police the local population and prevent it from rebelling, but also Arabs would eventually hear of this and attack them from the East while Byzantines were split.

It would take generations before Bulgarians would become loyal subjects. It's the kind of thing you can undertake when life is mostly peaceful and you're slowly chipping away at 1-2 weaker neighbours like Rome did, but it's not something you can do with a knife at your throat.

There was simply nowhere to expand in the West. Bulgaria was to the north, Illyria (modern-day Albania) was poor, full of mountains and angry hill folk, and simply not worth expanding into. They already owned a chunk of Italy that wasn't ruled by Franks.