Because the plants are reproduced asexually by cloning, exactly what you would expect happened, and a disease wiped out the monoculture fields (Gros Michel variety) of the early 1900s in the 50s and 60s.
Which we then replanted in purportedly less flavorful Cavendish, which was resistant to the then-dominant fungus.
Cue the new Panama fungal disease (Tropical Race 4) that Cavendishes are susceptible to, and we'll see what the future holds...
Maybe in 2085, grandparents will be waxing poetic about how they used to be able to get bananas everywhere cheaply.
It's fascinating to think that such a staple ("a banana") is completely different from the one people ate 60 years ago. And that they largely didn't exist in the United States grocery system 60 years before that.