It never had been a monarchy, really. "Prince" was just a title, it didn't connote being part of any royal family. It's hard to have a royal family when one is undead, after all, and live for centuries, and don't actually have children in the traditional way.

In the past, however, the previous prince had always named an heir. The vampire everyone had presumed to be his heir, however, was the one who killed him-- at his request, which kept everyone else from destroying her. But it did muddy up the waters enough for a couple of others to stick our noses in and demand an election.