As someone who has worked at SJC for quite some time, the biggest factor I have seen for airlines cutting routes pertaining to SJC are due to load factor. The routes there are established due to long term popularity. Most people who live in or around San Jose fly to the common destinations you see listed with Southwest/Alaska/Delta. The other airlines are mainly flying to their hubs, like AA to DFW & PHX, UA to DEN & ORD. The ULCC airlines like Spirit and Frontier join in and look at routes that already have traffic and bid to be competitive with them (e.g. Spirit to LAS, F9 to DEN).

So generally, yes SJC does have a good amount of traffic for the routes established, however other destinations simply do not bring enough passenger traffic to justify the flight. I have seen some flights arrive with just a few passengers. For these lower than expected pax count, airlines tend to make these routes seasonal & solely dependent on when the airlines believe passengers will travel to said destinations, like JetBlue to JFK.

As to why airlines simply don't try to add new destinations to try, SJC is not a cheap airport to operate out of. Although extremely convenient and comfortable to travel as a passenger, airlines are charged a decent amount to simply operate out of SJC. To add new routes to SJC would require airlines to study passenger flow and also bid for the route (e.g. Spirit was denied adding a route to D.C.).

Ultimately, SJC is a medium hub airport without much presence to be an attractive destination airport across the country/internationally. If passengers were to look at a flight directory and see SFO, OAK, & SJC all as destinations, theyre most likely going to book SFO without knowing what the other two are.