The military expansion under Justinian’s era, accompanied with the Plague of Justinian, lowered the population as well as stretching the Eastern Empire’s resources thinner with more territories to govern.

Even then, Sassanid Persia was on shaky ground and was devastated by these wars.

As a result, this created a perfect storm where its Westernmost territories are neglected while Egypt and the Near East fell under heavy taxation. Territories conquered under Justinian began veering away whereas Egypt was on the verge of revolt. With military losses and the rise of Islam, it was relatively easy for Muslim armies to take Egypt and the Near East alongside Persia, since they were done being under Constantinople’s thumb and Sassanid Persia was utterly devastated.

Had Justinian never expanded Westward or made a local king the Western Emperor for those reconquered territories (and then had it become independent) the financial state of the Empire,with or bereft of a plague, would have matched the Sassanids.

Sources:

Byzantium by Judith Herrin 1453: The War for Constantinople.... Lost to the West