The small area in the South you are talking about has more than half the State's population and is the largest constituency of the Colorado Compact. While the Imperial County claim is the bulk of California's claim to the Colorado, the water finds its way to cities as well. Thus California has a very large stake in the Colorado Compact (arguably the largest in terms of people and served and dollars made). If these people don't get water from the Colorado, they will look for it in the northern half of the state. It is cheaper and easier to collectively defend the very legitimate claims to the Colorado than to try to reallocate the state's other water resources.