I lived there too. College sports just isn't much part of the culture, in my experience, even with a lot of SDSU alums.
The argument that a new stadium/major conference would drive more interest doesn't seem to hold water. The old stadium closed after the 2019 season, and the new one opened for 2022, so we'll look at those years and skip over 2020 (obvious reasons) and 2021 (when the played in LA).
SDSU drew an average of 29,896 in 2019, and an average of 29,225 in 2022. Sure, capacity dropped from 55k at their old stadium to 35k at their new, but they had all of 1 game that drew over 35k (40k, Weber St, 2019). So having a new stadium didn't seem to draw much more interest.
Maybe if they were playing major teams? They drew 28k for BYU in 2019 and 34k for AZ in 2022. The AZ game was the very first game in the new stadium! After 2 years of "home" games outside of SD/no fans allowed, a regional P5 opponent and a brand new stadium and they still couldn't sell it out.
Comparatively, Utah has good fan support currently. ASU, AZ and CO have had good support when they can put decent teams on the field. SDSU has no history of that kind of support, regardless of who they're playing or how the team is doing, because it's not part of the local culture. I don't think playing schools located in states most San Diegans couldn't point out on a map