When Soul Calibur was in development, it was supposed to be more of a spinoff than a sequel. There were only supposed to be a few returning characters, though almost all of them ended up appearing in Soul Calibur 1. (Minus Li Long.)

Really, Soul Calibur just plays really differently from Soul Edge. That's enough reason to make it its own series.

Oh, also, the fact that Tim Langdell decided that he owned the word "Edge" and started suing everyone who used it in anything. :P