Strahd just resurrecting, and the world returning to the state as if the players never adventured there makes sense from the official timeline perspective
2E domains of dread explain how he comes back to (un)life - page 98
Vampires in general on 2E when reaches 0 hp doesn't get "killed", they assume "mistform" and have a chance to retreat to his Coffin. To permanently kill Strahd, you need to first find the location of his "coffin" which probably only he knows hence you need to somehow use control undead on him to force him to give the location of the coffin, ohhh wait, control undead doesn't work on domain lords since ravenloft messes with certain spells - page 289.
And even if you could, he has ridiculous high saves and with +2 bonus on his roll and the "power check" can corrupt or put curse on your PC atempting to take control over him. I an not joking. Even if it affected him, he would need to roll 1 to fail the save.( base save = 6 + 2 from necro specialization + 2 from how control undead works on ravenloft). And this assuming that this spell would work against him and that he din't planed any defense against this spell(which is extremely unlikely considering his high int)
"but I can use locate objects or wish to know the location of his coffin" -> Divination magic is weakened on the realms of dread and the wish will be "corrupted" by the dark forces. Wishing anything against a dark lord in the realms of dread is asking to be trolled hard by the
DM dark powers. You can also try to use his obsession with Tatiana, but doesn't matter how blind by love Strahd is. He will know that is a trap cuz no way that Tatiana would wanna ask him about it.
Permanently killing him is not easy. Even using "meta". Just defeating him and escaping is already very hard...
EDIT > To be clear(since people here love to put words into my mouth), I an not saying that wish, control undead and other spells of lv 7+ should be easily obtained on the Barovia. Nor that players should use meta knowledge. Just pointing out why he is so hard to permanently destroyed.