Thai food is by now fairly well known in many parts of the world and this is due to several reasons. Before explaining why this is so it is important to first say that the term Thai food is hard to define because there are so many varieties of Thai food to consider. This itself is due to several factors, the most important being the fact that the population of Thailand is made up of many different ethnic groups. Since all ethnic groups have food that is particular unto themselves this has given Thailand a very large base of cuisines to work with and defining a national cuisine is therefore a bit difficult.
Examples of different regional cuisines in Thailand include the deep southern dishes that have their roots in the Muslim-Malay cuisine of northern Malaysia, the far northwestern dishes that come from the Chiang Mai and Chiang areas with their hill tribe influences, the dishes of Isaan in the northeast that have Lao and Khmer roots and the many dishes in the central area of the country and in Bangkok that have numerous Chinese influences.
Thus, Thai cuisine is a true melting pot of influences, ingredients and cooking styles and this has created a national cuisine that has hundreds and hundreds of different dishes that draw on these varied roots. This is one of the reasons, as mentioned above, that Thai food is so well know around the world and so popular. There are so many different dishes and flavors associated with Thai cuisine that it always appears fresh and interesting to diners the world over. This variety undoubtedly contributes greatly to the universal appeal of Thai cuisine.
Another reason for the popularity of Thai food is the perception that it has of being healthy, at least healthier than most western cuisines. This is due mainly to the extensive use of rice and vegetables in many dishes as well as chicken and seafood. Pork is popular in many dishes too, and is also used in many decidedly unhealthy ways, such as very fatty sausages, but beef is not as prevalent. Also, many of the herbs and spices that are used have healthful qualities.
A final factor, although probably the least important, is the exotic image that people have in their minds of Thailand as a country. It is certainly not well known to many Americans, in particular, or Europeans either although there are many more visitors to Thailand from Europe than from the States. This bit of mystery about the country, its culture and its people must also be a contributing factor to the increasing interest in Thai cuisine.