I'm not aware of any special political reasons above and beyond the fact that politicians will always try to bring benefits to their constituencies. Perhaps others can comment if this is the case.

As far as I am aware, San Francisco was and is militarily significant primarily because it has an excellent natural harbour in a region that is strategically important. If you look at the west coast of North America there are few harbours of similar size and quality between San Diego and Vancouver and SF happens to be near the middle as well. This became important especially after American colonization of the Far East at the beginning of the 20th century and was dramatically amplified by the fact that the Pacific was an important theatre of war during WWII.

The base closures were due to military downsizing and consolidation. BRAC was the congressionally-mandated political process to decide where. As other commenters have noted, much of the Pacific fleet ended up in San Diego, but Seattle also had gains.

I think it's hard to say why the Bay Area lost out, but I wouldn't blame Dellums--all politicians try to maximize federal spending in their districts. A lot of the Bay Area locations were older, outmoded and in need of upgrades, hemmed in by civilian development, and more expensive to operate, compared to other west coast bases. Many were also crazy polluted (Hunters Point, Mare Island, Alameda NAS).

25 years later it looks like a good decision I'd say. There's new housing built or going up at Hamilton, Mare Island, Alameda, Concord, Oak Knoll, Treasure Island, and Fort Ord. Commercial development at most of the same plus the Presidio, Oakland Army Base, and Port of Stockton. And there's even been environmental restorations at places like Skagg's Island, not to mention the mountaintop Nike and radar sites, (although those places shut down long before BRAC.)