Why San Diego was the supportable alternative to Pearl Harbor

If Pearl Harbor was not a good base for the Pacific Fleet in 1941, where should it have gone?

San Diego, because it was the only alternative location able to support the fleet properly.

If Pearl Harbor had been judged unsuitable in 1941, San Diego was the only alternative capable of properly supporting the Pacific Fleet. Moving ships to a location does not mean that the location can sustain them. Strategic basing choices depend on support capacity as well as distance from an enemy. San Diego's value in this counterfactual rests on that logistical capacity.