Cause they killed each other off. That and actually I think that the Senju actually disseminated their bloodline (in theory Tsunade likely has Senju blood.) The Uchiha seemed like they were xenophobic assholes that probably inbred to keep the bloodline pure. We all know that Uchiha were one of the most powerful clans in Kohona before the massacre. Danzo and the Elders caused the destruction of the Uchiha (quite possibly under the influence of Tobi for unknown reasons.)
XI Wiki




