There are almost guaranteed others that are just hard to find, though as others have pointed out some are harder to ship. Go find grocery stores that caters to (West) African or Asian cuisine, and you'll at the very least almost certainly find plaintain varieties and possibly others.
But other than that, I expect it has a lot to do with what people expect a good banana to look like. E.g. Cavendish bananas tends to get removed from the shelves when they start to go brown/black, while plaintains are often still popular and sold until they've gotten much darker and look overripe and too soft for those of us used to Cavendish, for example. And we expect a certain consistency, and to not have to worry about e.g. deseed a banana - both expectations that'd be violated by a huge portion of the available varieties.
Carrots are in a similar position - there are huge amounts of varieties of all kinds of shapes and colors, yet the "right" look of a carrot is so ingrained that the few other varieties usually on offer tends to be sold almost as novelties.
They're not really biologically distinct. There are just some banana cultivars that taste good without cooking and some that taste better after cooking.
Not really true - as the other poster said, they are all part of a sort of species-complex. Some cultivars need to be cooked for hours, some for minutes, some are tasty without cooking.
But other than that, I expect it has a lot to do with what people expect a good banana to look like. E.g. Cavendish bananas tends to get removed from the shelves when they start to go brown/black, while plaintains are often still popular and sold until they've gotten much darker and look overripe and too soft for those of us used to Cavendish, for example. And we expect a certain consistency, and to not have to worry about e.g. deseed a banana - both expectations that'd be violated by a huge portion of the available varieties.
Carrots are in a similar position - there are huge amounts of varieties of all kinds of shapes and colors, yet the "right" look of a carrot is so ingrained that the few other varieties usually on offer tends to be sold almost as novelties.