Why did james buchanan and other democrats advocate a separate state in southern california in 1850?

They wanted to extend the Missouri Compromise line all the way to the Pacific. This would have split California into two states, one free and one slave. The state was large enough so that the state would have lay on both sides of the dividing line, and having the compromise line running all the way across the continent, west-to-east, would have been the best way to bring in more states in the future, in their minds.