Are you sure about this?

i.e. Tripadvisor says that AirAsia domestic flights are from Terminal 1, not the domestic terminal.

I'm taking my wife to Don Muang tomorrow, and trying to figure out which terminal I have to drop her off at seems to be complicated by a total lack of information on Air Asia's or AOT's websites about where to check-in. (i.e. I know AirAsia is terminal 1, but nowhere on their website does it say explicitly whether that is for domestic or just international flights).

AOT's website is even worse, you click on Pasenger(sic) information, and 3 of the 8 items are in Thai, and this is AFTER you selected English. Even ignoring the bad spelling, I think they'd have done a better job just pushing the web pages through google translate. And the pages on their website didn't appear to have any actual useful information on them. (I last used Don Muang a couple of years ago, and that was flying in only on Nok Air, not Air Asia.)

Found the answer - it seems all domestic flights were moved to terminal 1 in August 2011, so it would seem all domestic and international scheduled flights, are from Terminal 1.

http://www.donmuangairportonline.com/domestic-flights-shift-to-international-terminal-august

They said ALL budget airlines must use Don Mueang-- BUT Scoot( singapore air) use Suvarnapuhmn

WHY IS THIS SO.

very Strange.

Obvious really. Firstly, they are an international airline so its prudent to use an international airport, because passengers are connecting from say Australia to destinations world wide on International airlines. On my recent flight with Scoot from Coolongatta there were passengers connecting with 5 different european destination airlines. Have another cuppa and settle down !

What happened to single-airport policy? Wasn't that a key factor in building the corrupted Suvarnabhumi airport which took decades to complete. They should instead expand the airport and build another terminal building for low cost carriers. Just like in Malaysia and Singapore. These low cost carrier terminals are using same runways, within a near traffic free distance and could be built up within a year. The flood prone donmueng airport should be closed forever. Now it creates only more corruption opportunities and such a hassle to move between airports!

Probably a cost cutting exercise, seeing as how they had an airport just sitting there waiting to be used. It will probably grow e.g. Heathrow - Gatwick.