Simply put, Dracula isn't their king. As Spike pointed out in the Buffy episode he's actually pissed off most vampires by getting involved in the writing of the Bram Stoker novel, which did more to tell the public of a vampires weaknesses than anything else had managed before.
There's a fun Peter David "Spike and Dracula" miniseries which explains their particulat emnity, and includes a brilliant sequence where Spike and Dru go to see Bela Lugosi in a stage performance of "Dracula" and Spike is vastly more impressed by Lugosi than he ever was by Dracula.