Its because of ScuffGate. The issue was the way the iPhone 5 was anodized black on the back and sides (iPad mini, iPod touch and nano included, although the iPhone 5 had it the worst). The anodizing would rub off over time leaving the "bare" (aka, anodized, but with no color like the MBPs and silver iPhone) aluminum to shine through. I have seen this happen in person. After it happens the phone looks cheap.
I like the space gray, but it looks lighter on the iPhone then the other devices. It could be the fact it has the black glass accenting and antenna breaks, I don't know. But it looks better on the iPad and iPod touch/nano.
Also, all the colors this year look silver in the right (or wrong, depending on how you look at it) light, gold included. In different light, gold looks gold, silver - silver, and space gray, well, gray.