Simple colors, like green pumpkin, junebug, and watermelon, lack the detail a larger surfaced lure like a crankbait needs. If the whole lure is painted in a solid color with little to no contrast it gives the false impression that the lure was made on the cheap. Even though those colors would work, the consumer would think it was "too bland".
Another reason is hardbaits immitate different forage than soft plastics. Hard plastics almost exlusivley simulate baitfish of some kind or another, which have more complex color patterns (with the exception of crawfish which does give simpler color schemes). Soft plastics* that use monotones are usually imitating worms, crawfish, and creatures, which in nature are comprised of bland color patterns. So it's kind of self defeating to have a lure that moves like a fish but has the color scheme of a bug.
*swimbaits not included