I think we're talking about tourism. All flights from Ubon go to Bangkok. Krabi has flights to KL and Singapore. Hardly a comparison. Ubon isn't on most tourists "must do" list!
Yeah but with all due respect Ubon is an outback airport that once had a flight to Laos so it grabbed the "international" tag to pump up its importance! Krabi is the sixth busiest airport in Thailand
UK has a very bad DTA with Thailand. Fortunately us Aussies and the Americans as well, have a brilliant DTA which excludes pensions. Can't help you with this one!