In California, there are two ways for areas to be underserved by public four-year universities.
Firstly and most obviously: there are some relatively isolated parts of the state that have never had a public four-year university. Example: the Redding-Red Bluff Metropolitan Statistical Area (i.e. Shasta County), population 180,000. It’s about 75 miles from Redding to Chico State, the nearest CSU. In this case, Chico State gets credit for establish a degree completion program in Redding at Shasta College, the local CC (although it seems to me that it would be simpler to just allow Shasta to grant bachelor’s degrees).
Secondly and less obviously: there are parts of the state that were formerly served by accessible public four-year universities, but where access has been lost due to what might be termed “academic gentrification”. You know how downscale urban neighborhoods get “discovered” by hipsters and become fashionable, resulting in higher rents that push out the locals? An analogous development is occurring at some of California’s traditionally less selective state universities, like Cal Poly SLO, San Diego State or Long Beach State. They now attract tons of applications, and admissions standards have risen dramatically. Local B students that would like to get a low-cost education by commuting to their local state university are now getting turned away; they effectively have poorer college options than their parents did a generation ago. See, for example:
https://edsource.org/2016/rising-number-of-rejections-raise-fears-that-long-beach-is-becoming-elite-university/95431