Here's my speculation:
1 Case numbers are possibly understated in Thailand due to lack of testing. As for deaths, does Thailand have a robust mechanism for recording the causes of death and counting all the Covid deaths ? Also, even in western countries, reporting standards and ways of counting deaths vary. Some count everyone who dies WITH Covid-19 as a Covid-19 death, even if they may have died from something else and/or been about to keel over anyway.
2 It's possible that transmission of the virus is reduced in higher temperature situations. Although, I believe that's NOT the case for other coronaviruses.
3 Wearing of face masks has been more common in Thailand. Now WHO and medical advice on this seem mixed, but:
(i) Our authorities insist on mask wearing in hospital, so that implies masks have a benefit against airborne transmission.
(ii) The UK has a 2m social distancing recommendation. Now, if you think about that, it HAS to be for airborne transmission, as if there was no airborne transmission, the advice would just be not to touch people etc.
(iii) One by one, more countries are recommending masks. Even Singapore was late to that one.
(iv) There are scientific papers covering the subject of the virus lingering in the air on other particles.
4 I've seen reports of many different strains of the virus, some of which are more severe than others. This could influence country to country results.