Because they don't ship well-- skin thickness and ripening time. We can have them in the US and EU, but not at a reasonable cost.
Can't this be solved by improving the transportation of the less durable varieties?
The question is whether there is anything that can be readily made commercially viable and used for the role that we use the Cavendish now.
Most of the rest of bananas out there are not very much like the Cavendish we're used to (though some of the small ones are yummier-- if they could be made more hardy it would be worth getting used to eating tiny bananas ;).