Question

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

Answer 1

The short answer is that he overstretched his armies and ran out of cash by the end of his rule. The army was actually reduced from half a million soldiers to a quarter of that because he spent so much money on other projects. In the years after he died, most of those gains that had turned the Mediterranean into a Roman lake were lost to invaders.