Some possibilities:

* They don't know that their beans can actually sell for so much

* They don't know how to sell their beans for higher prices (e.g. they don't have access to the right middlemen)

* They aren't coordinated enough and undercut each other when dealing with the middlemen.

* Starbuck's premium prices may have little to do with the beans themselves

None of those would necessarily indicate that the farmers are acting irrationally in the previously described exchanges.