My guess is that Asians are usually hard working, busy oriented people, so they need some relaxation for their good work. Also, health is an essential thing in many Asian countries, particularly those that involves massage/spa. Many Asians believe that, not only spa and treatment/massage makes you feel comfortable, it's also very good for your health. In many parts of Asia, there are traditional doctors who use massage to cure a lot of things, from blood circulations to body temperature, and relaxing your tight muscles. As for the populatiry of Thai spa, I'm not sure how it became so popular, but based on many thoughts from Western people. Thai culture is considered paradise-like, so it might have something to do with the spa thing. People wanna go to a spa that feels like a paradise.