Most mountain bikes are relatively heavy (I know some are much lighter than others), and a rack is a big, heavy thing...mountain bikes with the proper mounts for a rack will tend to be entry-level mountain bikes which are especially heavy even by mountain bike standards.
I used to use a rack on my entry-level Raleigh mountain bike, but really it was so heavy it became barely usable. It felt like pedaling with stones tied to the bike. Your mileage may vary, as they say.