Preply are doing this as a survey to help their teachers, who can work from anywhere, to decide where to live. I think bkk is a decent city to live in. If you work from home then commuting is not much of a problem. On the edge of the city i rent a house with a lake and free fishing in the garden, that would not really be possible in other cities at the prices i pay.
There are nicer cities to live in but they tend to be much more expensive and there a cheaper cities but with limited luxuries and pishy unreliable internet. If i were to move from bangkok as a freelancer teaching online then i would probably go to HCMC or eastern europe as i think these are the next best paces to be a preply teacher