Indian American is interesting as in UK and Canada (don't know about Australia) average income for first generation is below average. But a lot of Indian Americans come on HB-1 Visa so high skilled jobs like tech sector or medicine which tend to pay quite well. In UK a lot came back when Commonwealth citizens had automatic right to move there so more working class. In Canada our recognition of foreign credentials is much weaker than US so you have a lot of those with medical degrees driving cabs in Canada.
I think the answer is that working class (by developed-world standards) Indians who emigrate mostly go to Commonwealth countries where they have families and other contacts who emigrated during the 20th century, which leaves only wealthy Indian emigrees to move to the United States. It's a reinforcing cycle, of course.
