Geography and politics.

Geographically, the heart of the Sassanid state, Ctesiphon, was close to Arabia on the Tigris River with little defences against attacks from the desert. The Byzantines also lost a lot of valuable land to the Arabs like Egypt and Syria, but maintained the core territory of Anatolia which is bounded in the east by the Taurus Mountains. To reach Constantinople, invading Arab armies would have had to penetrate the mountain passes, march all the way across Anatolia, and then have sea superiority so they could cross the straits into Europe and besiege the city. The Sassanids didn't have a similarly impenetrable redoubt.

Politically, the Byzantines and Sassanids had just concluded a 26 year long war (602-628) that ended poorly for both sides but moreso for the Sassanids. During this time, Arabia was steadily being united by Muhammed. The Byzantines fooled the Sassanid king Khosrau II into killing one of his ablest generals for treason, and Khosrau himself was murdered by his son in a bloody coup. His son then concluded peace with the Byzantines, but himself died shortly after. The Sassanids descended into civil war just as Muhammed's successors began to expand out of Arabia with a unified, theocratic state behind them. It still took the Muslims a couple of decades to conquer Persia (by the early 650s).

It was then the Muslim's turn to experience civil wars as the Umayyad dynasty established itself in in the mid 650s. This gave the Byzantines vital breathing room to reorganize. It's a complex process that's not entirely understood, but basically the Byzantines concentrated their remaining military strength in Anatolia and established military districts (called themes) based around each of their armies, which were then settled in the area. These districts eventually superseded the old Roman provinces and provided a large pool of military strength right on the border. While the Arabs would then attack Constantinople twice in the next century, they could not dislodge the Byzantines from these heavily militarized provinces.

Since no one has answered yet I will give you a brief summary of a few of the reasons/causes. I just took a course on the Sassanids and I can probably point you to some more in depth books if you like, until someone know knowledgeable comes around.

Simply put, the Sassinids, at the time of, and immediately before the invasion, had a much weaker state than the byzantines. I won't elaborate on why the Byzantine state was stronger, because I don't know enough to do anything but guess but this is why the Sassanids were weaker:

Firstly they were soundly defeated some time before the Muslims came by the emperor Heraklios, he basically made them a client state for a time and this disastrous defeat essentially shattered any unity within the kingdom. The Sassinids before this defeat had basically surrounded and almost completely defeated the Byzantines when the hail mary play was thrown, and it worked. The defeat completely embarassed the ruling classes, specifically the king, and the realm was throne into chaos for a long time. The ruling family and dynasty was almost completely wiped out, and for a brief time, temporarily usurped by a general. There were almost no male heirs to be found, so for a time a female ruler reigned, possibly furthering the erosion of authority of the monarchy. The constant wars for control and internal strife during this time certainly weakened the economy and population, not to mentino what institutions there were.

The loss of central authority also allowed the Satraps, the governors of the provinces considerably more freedom, and predictably some either failed to pay taxes/levies or openly rebelled because of the weakness of the monarchy. Furthermore in the south, there had long been an arab sattelite state that had been sponsored by previous rulers and used to raid the romans while saying "Nope wasn't us" cold war style. During this time, the ruling powers of Persia managed to antagonize these people, uprooted some of them and otherwise cut them loose. This sattelite state had served as a buffer against the wilder, further inland arab tribes. Those who once scouted, raided for, and defended the south-western tip of the Sassanid empire were no longer there, or otherwise allied with their enemies.

This long period of instability basically removed any real possibility of resistance in Persia. It could be said that the last reigning king, Yazdegerd, was well on his way to repairing the damage that had been done, but he simply did not have the time to enact those reforms and reestablish his authority. As a result when the first armies came, the Persian armies were poorly equipped, demoralized or disloyal. Many powerful nobles would flee to their other estates, thinking them safe, or would raise their own private armies and try and hold off on their own, or even tried to exploit the invasion to create their own petty kingdoms. In the end, we know this failed utterly and the Byzantines proved more resilient and hung on for another 700 years.