It's so clear that Thailand picked Sri Lanka because Sri Lanka is Buddhist and is one of the 5 Theravada nations in Asia (Thailand, Cambodia, Sri Lanka, Myanmar and Laos) that share cultural similarities. However, the thing is Sri Lanka is running of workers too, so why pick from a country that is importing workers too?
Thailand should pick from "blue" countries that have excess workers rather than pick from another country that can't lose more workers.