Great post @fitinfun.
I lived in Bangkok for sometime and for almost a year to complete a cooking course at the Dusit Thani (for French cooking, ironically, not Thai). Here are some facts and observations.
The reason street food in Thailand is strong is because for most working people, street food is Thai people's everyday food. Contrary to what it may seem, most Thai do not cook at home, and because their lifestyle is hectic like ours (leaving early morning for work/school, going home in the evening, stuck in heavy traffic), they buy their meals on the go or purchase food during a break.