Likely consistency of the product. The Cavandish cultivar is a clonal population; it makes then very susceptible to disease but also very consistent in terms of size and taste over time. The original Gros Michel cultivar was similar but then devastated by panama disease. It was also supposedly tastier and better.

But Plantains are also bananas and totally different in terms of taste and consistency. If you were to freely cultivate bananas there is a huge range of possible tastes and textures.

Thats not to say its not feasible; a range of cultivars could be developed. Its also worth bearing in mind that most farmers do not generally develop new cultivars; their business is mass producing the fruit. People experimenting with new cultivars are effectively fringe and it's likely difficult to break into a market where consumers expectations of what a banana is are so fixed. Most people don't even think of a plantain as a banana. Its hard to break in and make money with different bananas as people expect a banana to he a Cavendish.

Unfortunately consumers and retailers are the enemy of variety. Even fruits where people known there are varieties, such as Apples, are dominated by a couple of commerical cultivars. And that extends across into many plants and even animal products - there is huge commercial pressure for homogenous consistent products which in the shirt term out weighs long term risks of bad agricultural practicea.