Lack of refrigeration was another factor. Dried fish was available, but it was expensive. A large segment of the population is currently totally vegetarian, or may abstain from animal products for months at a time for religious reasons. A pretty big segment of our population believes that a vegetarian lifestyle is healthy. For many reasons, meat, fish, and dairy have not become a big part of Thai cuisine.
Vegetable oils are inexpensive and easy to obtain.
And, of course, for religious reasons, animal fat would not be acceptable to many people. Long beans can be found served alone or with tempeh. Tempeh and taro root are deep-fried together for a special entree, as are sweet yams. Cold green papaya salads are very popular. For dessert, Thaworn reminded me of all the wonderful fresh fruit that is available.