The floating markets in Thailand have developed over a long history in the region, likely developing before history is well documented. Much of central Thailand, including Bangkok, is located in wet lowlands of broad river valleys, all in a tropical climate. This meant that this area was heavy jungle prior to human development. The areas adjacent to the rivers were likely the first to be populated and the people living there would be using boats as their main mode of travel rather than trying to push their way through the dense jungle that once dominated the region.
Boats would have been used for both local and regional trade, bringing goods from those that produced to those that could buy or trade. Within the communities along the waterways, smaller boats were likely used to bring goods to those along the rivers, which included the majority of the people.
As the region grew in population and Bangkok began to develop into the areas capital city, the area had to be drained and this was done by excavating an extensive system of canals. While roads would have been built in this same time period, the already establish market system using the small boats would have remained the favorite method of distributing goods. So, the "Floating Market" would have remained ingrained in the Thai historical culture.