1. California is HUGE so regionalism is easier to establish.
2. LA is huge and a natural rival to northern California/SF Bay area.
3. The University of California system has 10 campuses. From that perspective, I imagine Cal Berkeley is as much a regional campus as Cal Los Angeles. The "University" Designation establishes the upper tier in California with the "STATE" institutions being the less prestigious: UCSB being harder to get into than San Jose State for example. Cal Berkeley and Cal-Los Angelese were in the major competing metro areas so were treated as equals.
4. UCLA got into the PAC-8 so that put it on equal or better athletic footing with Cal-Berkeley and USC. Not sure how it did that but I imagine demographics played a part: 2 Wash schools/2 Oregon schools/2 Northern Cal schools and 2 So. Cal. Schools. Nice balance and flagship locations.