Thailand did a great job contact tracing and isolating cases. Now we have no more local cases. Thailand tests seriously all people coming from abroad. Isolate and treat them. Who else should they test?
My friend works at Bangkok military hospital. No more cases there. They had over 100 at the pic
They don't want to import hords of new cases that will be impossible to handle.
So they limit the numbers. At the beginning at least. They select tourists from safe countries that will bring a lot of money and follow the rules. Backpackers from USA or Europe don't fit in that. Maybe later.