It's because all GMO efforts so far have failed to yield a banana plant that's sufficiently resistant to Panama Disease.

The most successful effort so far has been a Cavendish/Gros Michel hybrid, but it's only got partial resistance to Panama Disease, and is particularly susceptible to black sigatoka, a different disease affecting banana plants.

The main issue is that edible bananas reproduce asexually and most banana trees are direct offshoots planted from prior trees rather than a new generation of plant, so they don't breed the same way other crops do in order to select for more robust variants.