Ignoring the tremendous problems inherent in the term 'civilisation', you mention that the natives of the Southwest had complex semi-sedentary societies and comparatively large urban centers. This is entirely true of course, but we know that many of the natives of California were in regular contact with the Southwest and much of what is now Southern California was Sedentary at one point. The Cahuilla, Mohave, and Quechan peoples at various points all practiced agriculture. However these attempts at agriculture seem to have been limited compared to the Hohokam and other core southwestern groups. We know they were in close contact with the southwest as a whole, so it's entirely reasonable to ask why the social structures that enabled the large constructions farther east never migrated across the Colorado river.

What we know of the answer suggests that although many natives in California were aware of sedentary agriculture, they found other ways of life to be 'better' for themselves.

First of all the California groups I mentioned that flirted with agriculture generally tended to be foragers. The now-tame Colorado river was historically a very dangerous river. In years with high rainfall, it would flood frequently and for miles, destroying any structures in its way. On years with little rainfall, agriculture on the Gila, Salt, and upper Colorado rivers diverted much of it, leaving little usable water for canals. These factors made permanent agriculture in the Colorado river valleys even more tenuous than it already was. The Colorado desert between the river and the coast is also much drier than the eastern Sonoran, where agriculture was prevalent. When the area did become suitable for agriculture after the start of the little ice age (around 1300), we see evidence for early attempts at agriculture in the Imperial and Coachella valleys.

Secondly, agriculture is on its face a very bad proposition for foragers. It introduces tremendous land pressures that may not be compatible with local social organization, it only works in any given area for part of the year, and it generally requires tremendous amounts of labor as well as labor organization in comparison to foraging. None of the benefits of increased population density are as immediate as those significant drawbacks.

There is another, perhaps less obvious issue as well. California as it exists today is only a breadbasket because of how large it is. Central valley and the lower Colorado River give the state two opposing seasons (Spring/Summer growth and Fall/Winter Growth) in which to grow food. The state's size and long range trade networks enable it to switch between these two areas annually to maintain its food production. No such food redistribution network existed before the United States built up the territory. With the lower Colorado river largely unsuitable for agriculture until the construction of major dams, this system could not have evolved. The situation in Central valley was similarly problematic. Although the soil quality is fantastic, monumental canals are necessary to transport the water to that soil from far away. The human labor in this task is immense and the only people who knew how were far away in what is now Arizona.