I found that strange also. Why not Arlington with his Grandfather and Father? The Secretary of the Navy had to approve his burial choice given the requirements for burial at the Naval Academy as he really wasn’t entitled to it.
In fact I looked up who is buried there and I saw no past U.S. Senators, of course I only saw a select list.
I thought Flag rank was the minimum requirement for the Naval Academy cemetery. Maybe senator is equivalent protocol-wise, considering senator egos I can see it.