Why RGB instead of BGR, GBR or perhaps GRB?
Why CMYK instead of YMCK, KMCY, CKMY, YKMC and etc?
I presume RGB is in that order simply because it is the common spectral order — RoyGBiv, in other words. You will sometimes see BRG or otherwise when a particular computer image format happens to store the channel data in that order.
CMYK, on the other hand, is that way because that's the order the inks are normally applied in process color. See for example this article on four-color printing.