Also the Persian Empire was recovering at the time (poorly due to a succession crisis) from a rather long and brutal war with the Eastern Roman Empire.
There were seven great houses in the Sassanid empire and in the last years of the empire, they had been in conflict with the Sassanid court in South West Iran due to, according to some historians, years of overtaxing to support the war against Rome. Concurrent with the war, there was a plague in Western Persia killing half the population in 628.
When the Muslims invaded, the two most powerful houses refused to fight, and returned to their land in north east Iran. This left a weakened Western Persia alone to fight the invasion, which it could not successfully do.
When the Muslims invaded, the two most powerful houses refused to fight, and returned to their land in north east Iran. This left a weakened Western Persia alone to fight the invasion, which it could not successfully do.