Because bettas are from Thailand.
I purchase bettas from Thailand several times a year. I only had one bad incident with all the fish dying within a month from that one seller.
Thai bettas don't last long here because they go from large, cement tanks outdoors to our small controlled, tanks with water that isn't even close to how they were raised.
When I moved my bettas outside last summer the ones that did best were Thai bettas.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.