Because it's bigger.
HiRISE's aperture 0.5m, LROC NAC's 0.26m.

The Moon having no atmosphere allows for a much lower orbit so smaller telescopes will get you the desired resolution.
Saves a bundle.

HiRISE's job is to image the most interesting features on Mars, it could never image the whole surface.
LROC has to map the whole Moon in a reasonable amount of time, a higher resolution would mean having to take many more pictures.