There's a bunch of reasons but it's primarily the popularity of the airport combined with the runway layout and the noise abatement procedures all made worse by the Bay's weather.

SFO has two pairs of parallel runways and the pairs are very close together, with the runways in the 28L/28R pair only 750 feet apart. That's really close for an A350 with a giant 200ft wingspan and the noise abatement prevents the use of a standard step down approach where the pilots have much better visibility.

The two runways that run in parallel are too close to each other to land side-by-side in bad weather (because they only allow using Visual aka VFR landings).

"SFO operates on two sets of parallel runways. On fair weather days, SFO can accommodate approximately 60 arrivals per hour. During periods of low visibility, current FAA safety regulations allow aircraft to arrive side-by-side only if runways are at least 4,300 feet apart."

https://www.flysfo.com/about/airport-operations/policies-reg....