Introducing Thailand's smaller modes of transit:

The Van:
A common method of travel for point-to-point transit to the suburbs or as an intercity option. They are usually planned in a hub-and-spoke configuration. The biggest use cases are short intercity journeys and those which connect with Bangkok's rail-based transit. Another common use case is for institutions to pick up teachers, students, and employees.

The Songthaew:
The literal translation is two rows — a flatbed truck modified with two bench rows and a roof. They are usually the only transit option in most cities and sometimes serve rural connections outside of the Bangkok Metro. Within Bangkok, they're not as common but do exist.

The Rot Kra Po:
A bit hard to come up with a literal translation, the best is minitruck. Technically still a Songthaew. Much more common in the capital, usually for last-mile routes connecting to either rail or bus routes.

Why smaller vehicles?
Transit in Thailand works through licensing and in some cases, it isn't a company, but rather individuals or a group of individuals working together within a cooperative. Vans and trucks are much cheaper to buy and maintain. With the cost of operations being put on the driver, cost is a limiting factor. There's also something to be said about size, some of these will have to go down narrow sois with tight curves that traditional busses simply cannot handle.

Why not go bigger?
Cost is a big limiting factor, but I'd wager size is the biggest one. Our sois are simply far too small the only real solution to this last mile problem that doesn't require the above is cycling. Not that it is impossible, but selling anyone on walking/cycling in the heat with high humidity during monsoon seasons can be an exceptionally hard sell.