Thais have their own unique way of doing things. Once the norms are set up, they will all stick to them and not try to innovate. Same with running.
In Thailand, 5k's are almost always untimed fun runs with limited attendance. 10.5k's in Thailand are the equivalent of 5k's in the US. This means beginning runners in Thailand are usually thrown right into the 10.5k's, whereas beginning runners in the US tend to stick only to 5k's.
The US tends to have extremes where some people are extremely fit and others are extremely unfit, whereas in Thailand many people seem to be at about the same level, with few extremely fit and extremely unfit people. The general public is happy to jog ~10.5k and seem to love that (approximate) distance. No one seems to complain about inconsistent distances. They just want to get out and have fun.
There is something called a "mai pen rai" mentality here, which means a "whatever goes, take it easy" attitude where you aren't nit-picky about little things. This can translate to a lack of attention to detail like race distances.
Thai mini marathons are where you see the most smiles and silly faces being made to photographers. Hardly anyone looks like he is in pain or struggling. This is a hobby jogger's paradise. If you are a serious hardcore runner, you will really annoyed by this supportive, happy-go-lucky, take-it-easy fun jogging culture where people seem to lack ambition and competitiveness. But that's just how people view life here.
There is a small elite running scene, but there are no more than a few dozen fast runners in the country who keep showing up to the same races over and over again. They all know each other and it's a tiny world for the elite guys.