It actually continues to surprise me how low yielding SJC is compared to SFO. You'd think some people from Silicon Valley would find SJC transcon flights more convenient.
The situation in the bay area is quite interesting actually. Yields are all directly related to the cost of real estate in the vicinity that each airport serves (which makes sense). The peninsula+sf has the highest housing costs, while the south and east bay have the lowest - and the population is starting to gravitate towards those areas. So while San Jose has a large catchment area that is quickly expanding relative to other Bay Area airports and especially to SFO, it is low-yielding because it's the population center for your regular Bay Area person. Meanwhile SFO serves the international audience and the techies - and it's the only facility that has the infrastructure and space to support proper lounges, connectivity, etc.
However, there are many luxury developments going up both in the heart of San Jose as well as in the hills both east and west of the city. These will likely lead people with higher disposable income to use SJC more, but it makes life a whole lot harder to use SJC when you are trying to book a flight to Europe, Asia, or even the east coast - especially if you are a FF or have any kind of loyalty whether it's to OW/*A/Skyteam. Plus of course there is the immense prestige factor that many people are going after by living in peninsula zip codes.
A little history lesson on the present and future of the bay area, LOL.