Not just disinterest, but competence. The skills that make you a good writer are very different than the skills that make you a good marketer.

My brother in law, whom I love dearly, self-published a novel. He's gone to all sorts of local fairs in order to sell it. But when I ask him for the elevator pitch, he can't give one. I've listened to him talk about it, and he can't even get ME interested in reading it.