I see it from a different POV.
The Challenger and Challenger R/T were targeted to different markets, although there was overlap between the two.
What was considered desirable colors for a hi-po car was different than something more pedestrian. Plus, our idea of colors for hi-po cars back then was completely different back then.
For example, black was a formal color often associated with prestigious 4-door sedans; it was not a popular color among hi-po cars (at best, it seems middle-of-the-road). Today, black is probably #2 after red for hi-po cars, which prob explains why it is so popular among collectors (and also explains why we eschew green, which was so popular back in the day but those greens haven't aged well to contemporary eyes).