At a random guess it's due to sanctions. You could as a last chance try a specialist travel agent like Propeller travel (often cited on here, I have never used them) and if they can't book anything maybe contact a traditional travel agent in Rangoon and see what they can do?

https://propellertravel.com/

Possible agents in Rangoon


Eg. Sun Far Travel

The advantage of talking to a local agent is that they will.know what does and doesn't work. I wonder whether making a payment using a foreign card might be troublesome though.