There are differences, probably geographical as well as cultural, that counteract the similarities.
Seattle has rural areas a lot more closer to the urbanized core. If you drive out to those areas -- Arlington, Enumclaw, Maple Valley, Duvall, Fall City, Monroe, etc., and even south of the metro where the FM signals from Seattle still reach, you see more farms and agricultural areas, and chances are high that if you hear a station being played in a store or on a car stereo it's country. There always have been country music fans in the city and suburbs as well. But the proximity of the rural areas is a lot closer than you see in the Bay Area. Enumclaw and Fall City (the Snoqualmie Valley) are 30 miles or less from the urban core. In the Bay Area, the closest rural areas are probably 50-60 miles away.
Also, today's country music has a lot of pop and urban influences included, so it's not as 'rural' sounding as it may have been in the 1960's or 70's. So a lot of the difference between country music's popularity in SFO and Seattle-Tacoma may actually be just cultural. After all, Seattle has a longer history of being isolated from the rest of the country than SFO. Up until the 1990's, Seattle was more of a backwater. Can't say that about SFO, even in the early 20th Century it was a vital urban center in the US.