Does anyone know why the "upscale" grocery stores in the US (e.g. Whole Foods) don't try to sell other varieties of bananas? It seems like they could charge quite a premium for the "exotic" bananas not available to the people of Walmart.

Just a guess, but every non-cavendish variety I've seen in southeast asia is much smaller. Haven't tried them though. Maybe they taste better.

some upscale markets in Brazil did have the south Asian varieties.

They were darker in color (reminding a month old regular banana in some cases and reddish in others). much smaller. less sweet. way more expensive.