Basically, this is because cavendish bananas travel well (they are relatively thick skinned, and ripen while traveling reasonably well) while still being reasonably tender and flavourful as a raw fruit (as opposed to other bananas with thick skin that are usually cooked or processed).
The high diversity of apple cultivars available in North America is a combination of basically all apples having relatively sufficient storeability and shipability, as well as the fact that apple trees will happily grow across most of the United States / densely inhabited Canada. They are effectively "domestic". Bananas have a far more limited range in the US (mostly limited to Florida and Haiwaii).
Breeding new strains of banana is also much more challenging than apples since... well, the bananas people want as a raw fruit are seedless. You're working with trying to hybridize populations of clones. This is understandably more challenging than dealing with apples.
Sugar bananas and Ladyfinger have their own place. Indonesians seem to do sugar bananas in a caramel sauce which I hoover up in obscene quantities at hotels. I like both a bit tarter and crisper than a Cavendish.
Banana flour is a digestive miracle high in inulin or inulin producing precursors and keeps my lower gut happy.
Winnowing down to (for the west) basically one sterile cultivar was a huge mistake.