I don't recall them ever saying why, but Im sure they just think it's cheesy. I wouldn't buy a colored iPhone. You can get them colored, if you're willing to pay a PRETTY penny. Over $1000 for sure.
they have tried white and the word is they are now trying pink. the problem with the was the home button did not match the white glass. and you can get the entire phone changed to white for less than $300. the $1000 figure is way off!
There are many cases available in the market, but the real manufactured piece is a real charm to get, I have seen a white one but black color on the body still rule.