Because unlike the UK, Europe, and the US - Thailand doesn't offer the same "incentives" to come and live off the Thai Taxpayer like they do in the West.
Also, migrants would be competing against the Thai labor force which is already highly protected under Thailands regulatory scheme. Thai authorities and politicians are all to familiar with how Thai citizens would take that. As in, it would go over like a fart in Church...
Last, the rape, violence, and mass protest you see being perpetrated by migrants in many countrys would receive a very different, and imo highly reflexive response in Thailand compared to what you are seeing in the West. Again, Thai authorities and politicians have enough drama without importing more.