The difference is the lack of AS competition and much lower volume to PDX when you look at the larger market, which is Bay Area to Southern California. SFO-SNA competes with OAK/SJC-SNA and, depending on destination in SoCal, SFO/OAK/SJC-LAX/LGB/ONT and perhaps even BUR and SAN. Between those airports there are hundreds of flights every day between WN, B6, AS, AA, UA and VX. SFO-PDX is a thin route with competition from OAK and SJC in the Bay Area and no real alternate airport in the PDX area. Even though AS, WN and UA fly the routes, they seem to have achieved a decent balance that has allowed them to keep flights full and, based on the prices I am paying for so-called cheap fares on AS, decent yields. Entry by VX would likely cause AS to start a fare war, just as it did on SFO-SEA. I suspect VX wants to time that entry just right to ensure that it can survive the fare war.