This is not something unique to the Chinese and Southeast Asia.

Another good example of this include would be sub-Saharan Africa in which a whole host of immigrant ethnic groups and countries, such as the Lebanese in Kenya, dominate the economic sphere.

Amy Chua (Yes, this is the same person who wrote about tiger moms but her prior works were a lot better) in, World on Fire, proposes that this is because of crony capitalism. Essentially, the ruling class needs someone to run businesses, however, there exist an inherit tension between moneyed interest and political power. Not only that, but the ruling class would often need to extract wealth from the business class to finance either state activities or personal corruption.

Therefore the logical solution is to have something very much like the relationship between European Kings and Jewish moneylenders in the Middle Ages. If the business class are natives, then there would be a backlash against expropriating their property, and wealthy businessmen are a political threat. However, if the business class are in the hands of immigrant minorities, then this isn't an issue. The immigrants would be hated and despised by the local population, and hence dependent on the ruling elite for protection, giving the rulers leverage over them. They are incapable of taking political power, and wealth can be extracted with far less consequence from this group of people. So it's a pretty big win for the ruling class and (with significantly more caveats) for the immigrant minority in question.

The specific examples that Chua gives for SE Asia are the Chinese in Philippines under Marcos, and in Indonesia under Suharto. Under both regimes both minorities were able to capture a significant part the economy through collusion with the ruling, dictatorial elite. However, the democratization of both countries have placed those minorities under significant threat (see anti-Chinese riots in Indonesia) and their futures are very much in doubt.