Why did Justinian I fail to regain all of the former provinces of the old Roman Empire?

Question

nurali · Answer

i think He ran out of money.

anonymous · Answer

Justinian the great was probably the last of the Romans he reconcured most of the old empire, and this increased his reveunes (money), it was probably the plague, bubonic, I think, that actually stopped him.

anonymous · Answer

Nurali is right, Justinian's armies pushed as far as they could and the empire reached its largest extant under his rule -- but he was terrible with money. 

By the end of his reign, the treasury was drained and the army had actually shrunk because he couldn't pay everyone. It's also another reason why many of those gains were lost after Justinian died. His successors were unable to hold onto them either politically or because they just didn't have the manpower (and money).