Because Apple compared it to the Air. It was basically advertised as having all the features of the Air.
It does not.
To which I, who is very happy with my rMini, and is an owner of an Air with a more problematic screen, say so forking what?
The Mini skimps on some of the colors you get with an Air. It also skimps on the size, weight, and price over an Air.
It is what it is, and what it is is pretty great. If you absolutely must have a full range of colors in your tablet, the Mini is not for you. If you can accept the fact that it makes a slight sacrifice to bring you great battery life and portability, and that you will likely stop thinking about the color after about a day of using it, then go for it.
The best proof that the rMini's color gamut "issue" is rather overblown is that the prevailing thread discussing the topic, which grew to 13 pages when the rMini was released, has now fallen to page 8 of this forum, with the last post made 11 days ago. 11 days on Macrumors is about 6 months in real time.
Given that even more people now have rMinis in their possession than 11 days ago, yet the main thread discussing gamut has gone dormant, one can infer two conclusions; 1. The bulk of the grumblings were driven by fear over experience. People were all up in arms about a screen they hadn't even seen with their own eyes yet. And 2. The problem has proven to be a non-problem for many if not most people who now actually own one, in this otherwise EXTREMELY picky community.