1) Outside of the present day DoD bureaucracy which has been culled and groomed to reflect Obama and Biden since 2008 (Trump didn't fully realize the importance of this in his first term). His lack of experience beyond that is actually a plus because it makes it less likely that he is another Deep Stater, and also someone who hasn't been formated into the same way of thinking of the past 16 years.
and
2) He will be loyal to the Chain of Command - a really, really big deal to a POTUS who had his top general going behind his back with our greatest enemy and get away with it.
Trump wants a reliable head at the DoD. It is his right to choose the person who he thinks can best give him this. Despite the idiots here who expect that he should be required to pick another Obama loyalist who wants to use the military as a giant social experiment rather than as an entity that can wage wars effectively when needed.