The ordering is mostly conventional rather than a strict technical requirement.

For RGB, the usual explanation is that red, green, and blue are named in the familiar visible-spectrum order people commonly learn (as in “ROYGBIV”). In practice, some file formats or hardware may store channels in a different order, such as BGR, but the color model itself is still commonly referred to as RGB.

For CMYK, the order comes from printing conventions: cyan, magenta, yellow, and black are the standard process inks, and that sequence reflects how process color is commonly described and often how inks are applied in printing workflows.

So the names RGB and CMYK are mainly standardized, widely understood conventions based on common spectral and printing usage, not because other channel orders would be impossible.