A lot of it had to do with the circumstances of the war. Phocas had taken power in a bloody coup in 602 (overthrowing Maurice), which plunged the Empire into, if not civil war, civil strife. Because of familial alliances between Maurice's family and the Persian royal family, the Sassanid invasion was initially pitched as an at of vengeance. Basically, the Empire couldn't get its shit together right awa, then you have the overthrow of Phocas by Heraclius, and by that time it's 610 and you're already losing all across the empire. When you lose Palestine (614) and Egypt (618) most of the support areas for the state go away (Egyptian grain, Gaza wine). This is also of course an empire that is already weakened by successive waves of bubonic plague.

The best book on this is Walter Kaegi's, but Haldon's The Empire that Would Not Die touches on some good stuff as well