Do you actually know that the ePub owners would be willing to licence their format to Amazon? They can't just add ePub to a kindle, they need to licence it and pay a fee for every device. They most likely would also have to pay Adobe too as not all ePub are interchangeable.
And the same could be said about those who choose ePub. They could have licensed amazons format but just as much want to keep their customers away from Amazon. Amazon most likely would have been happy to take get a fee per device sold and a fee per book to supplement their existing income as well as making it easier to sell books themselves to others seeing as they make very little on the hardware.
Isn't ePub a free and open standard?