Insecure North American gamers care a little less about other colours. I'm sure some do but it's not a big a deal as in Japan.
And to meet demand, it's probably slightly easier to mass-produce DS Lites in just one colour, which is why it's only officially/non-importingly available in white.
Plus they get slight sales spikes each time a new colour of something comes out, so they space them out to maintain interest in it. North America will probably see black DS Lites when DS Lite sales slow down...which could be a while.