How can a rapper/emcee who sold a million copies of an album be underrated?

Heavy D. was big from his first single, first album, and was well known in the rap industry by time his second album dropped, so no he's not underrated at all; most would consider him old school rap artist since a lot of the new generation wouldn't really know of him since he kind of fell off and went back underground by his sixth album (first solo album) was released but that's expected, not many artists who have been in the game as long as he has most don't think they would continue to have the same success music & name-wise for that long but he had a good run.

To be underrated you needed to have an album or single that never sold big (meaning didn't go platinum), but was well known by the underground scene but not by mainstream media, a rapper/emcee who was very lyrical and could make music that all coasts (east, west, mid-west) could get behind if one of his songs was played, if they spat dope verse, etc.