National Parks are intended to protect and preserve some of the most precious natural areas in the country. Campgrounds especially modern ones have a huge footprint. Installing and maintaining the utilities mean digging things up, bringing in heavy trucks, etc. Also where does the waste go? Has to be tricked out, more big trucks.
It's bad enough that any camping occurs inside the parks. Seeing something like a RV resort with hundreds or thousands of sites sardined next to each other built inside a park would be awful.
Let private developers build those abominations outside parks if they must.
