I had never thought about this until reading the OP's inquiry, and can't offer a definitive response, other than to suggest that the remote locations of the major National Parks, at least in the west must be a contributing factor.
Here in the east, I know of at least three that are reasonably close to Marriott properties; Acadia in Maine, Valley Forge and Everglades. There is also Gettysburg, but I am not certain whether it is a US or state site. In either case it is worth a visit.