The parallel runways are too close together for simultaneous ILS approaches when it's foggy, and it's always foggy.
This halves the landing capacity.
OAK is closer to downtown and much less delays even though there's just the one big runway because there's less jet traffic.
SJC is still further away but much less delays there also.
I remember at the time Southwest said that it just made more economic sense to move out of SFO for those reasons.
When I was in CA United were pretty much the main game in town at SFO. Don't know what it's like now. Are you starting a Low Cost Carrier?