There should be more solar panels in Thailand as there seems to be solar rays every day. However, it's commercial use is very limited, at least, that's from observation. It will definitely reduce the electricity bill in the long-run for the ordinary Thais. Maybe it's the initial start-up cost that disinterest them. You said in another comment that it doesn't need to be expensive, so maybe it's not as widely promoted over here.
The price of solar panels is coming down rapidly, at least in the US and Europe. It would still be a lot of money in countries like Thailand where the average monthly salary is just over $400 (with most people making much less).
You know your figures! Yes, it's quite shameful that the average monthly salaries are low as Thailand strives to be a leading force in ASEAN (Singapore leads the way - stark contrast in average monthly salaries with them). Still we are sure there are projects from corporations to install solar panels in low-economic regions to help the locals.