This had as much to do with the quality of the displays as it did with the adapter. White, black, cyan and magenta all have significant contrast between them, a

This had as much to do with the quality of the displays as it did with the adapter. White, black, cyan and magenta all have significant contrast between them, and many machines with CGA were still hooked up to B&W and greenscreen monitors. If you'd designed your graphics around Mode 0 (the yellow, green, red, black one), it would look really terrible on a greenscreen. Even full colour displays (often TVs) would look miserable in the other colour modes, as they'd bleed much more than displays do now. The white/cyan/magenta/black was also really easy to build as a secondary palette for 16-colour EGA games. I remember a lot of the Sierra adventure games at the time offered a CGA mode, and while it never looked nearly as good as the EGA mode, it was at least playable.