I just moved from San Francisco to Nashville, TN last week. For me, it was a combination of the cost of living and lack of family in the area. I'll always miss San Francisco, but our rent has halved, our square footage has doubled, and I no longer pay state income taxes. I'm still doing interesting work (remotely) for the same company, so I can't complain!
Genius is a dynamic thing rendered via the lens of social value.
If scientific progress is our goal, then I would argue that forming cliques around a clique-governed definition of "most likely to elicit genius" is not only wrong, but dangerous. Heuristics can be ugly things.
Also, MENSA reminds me of church. Church is boring.
My intuition tells me that neuroplasticity is largely driven by genetics. Outliers in this space are likely caused by mutations related to neurofibromatosis. I also imagine that there is a huge memory component in some of these variants (for example, a person who can learn quickly but also forgets quickly).