I don't know if it was ever explained fully in canon, but here's my theory - Buffyverse Dracula isn't really the kind of the vampires, he's more like the Elvis of vampires. And compared to the various Big Bad's they've fought, he isn't a huge threat to the world - he's not opening Hellmouths or trying to wake up Earth-swallowing demon rocks. But he's still way stronger than the average vampire or demon, and it would take a reasonable amount of effort to kill him, especially on his own turf. If the issue ever came up, they probably decided they had more important things to use their resources on. Especially since they started being attacked by people actively seeking the destruction of all slayers.