Back in the day, I think most of those colorful keycaps were more about brand association, using the brands colors to stand out not so much for any usability. A lot of brands had thier colors and used them for everything, you know what they are before you even see the logo.
Though, I don't disagree with you it dosen't hurt to mix it up and offer something besides white, biege, grey and black. If you look for them, there are some options out there though, just harder to find limited edition stuff mostly.