It's a fair enough question, and there are a number of reasons for why Ctesiphon became the Sasanian capital. Some of them are geographical, some are historical, and some are cultural. The most important reason is probably the following:

• The Sasanians were a successor empire to the Arsacid (or Parthian) Empire, which had its capital seated in Ctesiphon. To make such a transition as smooth as possible, it is very often a good idea to run the administration from the same capital. But, of course there is more to it than that.

First, it is important to consider what "Persian heartlands" would have meant to the Sasanians. Their heartlands would have been the region of Fars (or Pars, in pre-Arabic times), a mountainous region in the southwest of today's Iran. The Achaemenids, five hundred years earlier, had actually constructed a capital there, usually called Persepolis, which was apparently destroyed by Alexander. But even then, for the Achaemenids, Babylon remained their administrative capital. Why is this? Well, there are a number of good reasons. This brings us to what we might call geographic or economic reasons.

• Mesopotamia was fabulously wealthy in antiquity in terms of the amount of grain it could grow, rivalled only by the Egyptian Nile Delta in the region. The administrative system established by the Achaemenids and continued under the Sasanians, established satrapies, where the Great King would appoint a local governor to rule over a region. The Satraps answered to the Great King and the nobility, of course, but could exercise significant independent authority. Generally, it is simply a bad idea to have vassals ruling over lands that are wealthier than the crown lands and which might supply a significant portion of your empire, in case they turn rebellious.

• In comparison, the geography of most of Greater Iran is pretty harsh. The most fertile regions in Iran proper (i.e. roughly the modern state) are fairly thin bands of mountain steppe in the west and north that are somewhat suitable for establishing cities. It's a huge place, but much of the land is desert. Within the territory of the Sasanians, the other option would have been to settle down in Greater Khorasan the irrigated steppes of what is today mostly Uzbekistan and Turkmenistan, like the Khwarezmians would a thousand years later. When you consider that the most significant rival to the Sasanian empire was the Roman Empire to its west, it's just not a good idea for reasons of logistics to have the king be on the far side of mountain ranges and/or deserts, especially not if your most wealthy region lies right at the border of your rival.

So to sum it up, there was no shortage of reasons to settle in Mesopotamia. It was the capital province of not just their immediate predecessors but also their esteemed ancestors, and of other kingdoms and empires since time immemorial. It was incredibly wealthy, and it was vulnerable to attacks from their rivals.