The regional Mexican stations don't do as well due to the higher percentage of Hispanics who are not Mexican or Mexican American.I think it's also possible that the format doesn't connect in the Bay Area for whatever reason. Univision's Latino Mix simulcast in Dallas-Fort Worth also has some signal gaps in the market, but Latino Mix performs better over there than it does in either San Francisco or San Jose. However, It's quite odd that not even the full signal Regional Mexican stations in San Francisco do well despite the market's high Hispanic population. I definitely wonder why the Spanish stations in San Francisco don't do as well as the Spanish stations do in other markets with a large Hispanic population.
The SBS station has a mediocre signal in the San Jose are, and the two Univision parallel signals have weak areas. The other two UVN stations have huge areas not well covered, so there are a variety of factors limiting those stations.
Remember, for ratings San Jose IS San Francisco as it is an embedded market. The SF book includes the San Jose sample.