They're not close substitutes. If you want or need to go to BKK, are you willing to fly to SFO instead? Probably not. The trip to Thailand could be for a specific conference taking place in Thailand, tourism (specifically to visit temples, palaces, and shrines, see the floating markets, and to take a river cruise), medical tourism, sex tourism, to explore Asian cuisine, or for business meetings in Bangkok (to visit customers or a manufacturing site). Obviously, none of these objectives can be completely fulfilled easily in the San Francisco area.