The reason for it is sparse population, not so much lack of control over the region. I read some numbers that Panonian Slavs numbered to 500K when Magyars arrived. There might have been other inhabitans (remains of hun and avar occupation). Not to say that Moravia had a complete control over its conquered regions (it was still in transition form tribal to state in peripheral parts of the empire). You'll hardly find a fairly centralized state in Europe at that time. One thing I would like to know is the size of Magyar invasion. I would like to know the numbers accompanying them.