Why did justinian i fail to regain all of the former provinces of the old roman empire

Because the east had fallen into what was called the " dark ages " many barbarian tribes ran a muck in the eastern empire after the fall of the "roman wall" holding the north barbarians from entering eastern Rome, many from the east tired to feel west to the Byzantine empire, many also did not make it.

That really sucks, but i don't know who your leader is, and i don't want to write that much