This is because Cook is a pipeline guy, not a products guy, as Jobs himself stated as a concern.
Jobs picked Cook because he knew he was the best bean counter and there was no way Jobs' product line in the works would bankrupt the company with him managing it. To say Jobs thought he was a product guy or had a vision would be stretching it. Keep in mind, Steve Jobs never really thought he was going to die! He totally believed his crazy homeopathic treatments would cure him better than modern science.