The shortest and simplest guess is that the more the colors, the more the cost. I know that with painters that blend their own colors (be they fine artists or house painters), they will often start with primaries, so that would lead me to think the primary dyes/colors would be in the highest demand, thus the most plentiful and cheapest as a result, hence the preponderance of reds, yellows and blues used by printers with perhaps the occasional green or purple.
I know that if you see an ink used for linework that appears brown, it may well have been black at the time. Iron was used in many early black inks, and as the iron oxidized over the years, it turned from black to brown, hence the reason so many older documents appear to be written in sepia rather than black inks. (Oh, the things we learn on the History Channel...)