Houston has mission control, lots of astronaut training, and a lot of administration in Houston. But Pasadena CA has the Jet Propulsion Lab, which is a big part of the design of the rockets and robots and what not.
The Los Angeles area in general has lots of aerospace engineering: besides JPL, Lockheed, Northrop, Boeing each have a big presence, not to mention the plethora of smaller defense contractors.
I lived in Lompoc, CA for a while (next to Vandenberg -- which is a really cool experience, the rocket launches were absolutely amazing). I had always assumed that the El Segundo presence was in big part due to access to Vandenberg for launches.