@mcc
I think it's more technical than price. Remember these are 8 bit machines, so it's easier to express colors in ways that fit inside a single byte, otherwise you have to do three instructions per pixel, which slows your calculations down significantly.

That limitation gives you 2 bits per color (or 2/3/3 if you want to get fancy), which means you do not have that many options for color gradients.
@aeva