Food and beaches are hard to beat for sure, also the islands and nature, culture and brilliant nightlife and whatnot, but probably one of the main factors why Thailand has an upper hand when it comes to being a popular tourist destination in SE Asia is its superior infrastructure (close or equal to "first world") and highly developed tourism sector which caters to pretty much everyone's needs and budget. Another factor is stability; it has never been colonised and other than a few coup d'état that were more or less bloodless, the government and especially the Royal family kept the tensions in check and the country remained stable throughout the years whereas its neighbours suffered numerous civil wars, dictatorial communist regimes, economical shortages etc. This stability as well as their openness to foreign businesses starting from the 60s led to an exponential growth and that's when the country started to appear on the travel magazines as an attractive destination with a great cost to quality ratio.