Similar to IAD. It is a key hub for international connections. So you have a # of paid F seats on domestic flights that are taken right off the bat by corporate travelers flying TPAC in paid C international. Similarly SFO is the major airport for high tech and other O/D traffic so again you have paid C flying East or South in F to connect to TATL or South America flights. So you start with a lower # of remaining seats for upgrades. You can see this by checking the seat maps at T-7 days - many seats already gone in F.

Then you take the huge # of GS in SF, not to mention 1Ks, and the result is that every hub-hub flight has dozens of elites in the cattle car. Your only real shot is flying at a real offpeak time or to a 2nd-tier destination.