In the broader cosmology (i.e. digging back to prior editions), Ravenloft is just one of many Demiplanes of Dread. Each of them is the Domain of a Dark Lord. The Domains are created by the Dark Powers as a customized prison and punishment for each Dark Lord. What the Dark Powers are or why they create these grand torture chambers is deliberately left unknown - they simply are and do.
The torture the Dark Powers inflict is based on the "crimes" of the imprisoned. Some Dark Lords have more or less power over their Domains, as it suits the nature of their punishment. Even player characters are tools of the Dark Powers. They're brought into the Demiplane as needed, used to push forward the Dark Powers' goals, and then released ("allowed to escape") back to where they came from when their usefulness ends. The Dark Powers actually care more for their chosen victim than they do bystanders. Indeed, quite often the innocent make up part of the punishment.
In Strahd's case, both his continued immortality and his pursuit of Tatyanna are part of his punishment. It is the Dark Powers that bring him back every time he is killed, despite anything characters might do to prevent it - the Dark Powers control the Demiplanes of Dread, nobody can truly challenge them. The Dark Powers use Tatyanna's soul as an implement of torture, compelling Strahd to hunt for her, find her, and lose her over and over again... so they bring that back as well. Whether it is the actual soul of the original woman, not allowed to go free (despite appearances), or a duplicate perfect enough to fool Strahd is unknown and irrelevant: it functions the way the Dark Powers need it to.