There are two big mountain ranges crossing Mexico North-South. That probably kept some of the indigenous groups isolated from each other for a long time.

There is also the division between Mesoamerica (South) and Aridoamerica(North). Two different sets of cultures that needed different technologies. Settlers and Nomads.

That may be so but the take away should be it's not just intuition but it's documented and contributes to scientific knowledge and inquiry and may lead to direct improved knowledge about those pops leading to better medical understanding and treatments... And no just for them but other genetically divergent groups around the world.

Why are the islands known to accelerate genetic changes specifically? Is it because of sun exposure?

Isolation. See The Galapagos + Darwin's studies on the birds there. Also: Madagascar (film + land mass)

From what I've read about animals, the isolated population of an island results in faster evolution due to 'close' breeding. Rare non-dominant genes are more likely to meet up and be expressed. The lack of geographic diversity also means that species need not evolve to be generalists. There is no advantage to adapting to anything but the island environment. In humans, by way of example, that might mean an island population more rapidly adapts to an endemic disease as they cannot escape it. They don't have people living elsewhere brining in non-adaptive genes and so the local adaptive genes spread more quickly than if they were on the mainland.

I Googled "islands accelerate genetic changes" and the results show stuff like:

The restricted scale, isolation, and sharp boundaries of islands create unique selective pressures, often to dramatic effect.