You can't transfer from a Cal State (CSU) campus to a University of California (UC).

My son attended CSUN for the Fall Semester of his Freshman year with hopes of transferring to UCLA or UCSB, CAL, ans UCSC. We quickly found out that UC transfers from Community Colleges and JC's have preference and some campuses of UC don't accept any transfers from Cal State students. In fact, 97% of transfer to the UC system come from CC's and JC's. Just Curious if anyone knows WHY that is ? Are the CSU and UC systems academic rivals ? Thanks