>>5055491
It all comes down to a "first step" problem.
For example, white animal evolves to be black because of soot. This is understandable: either it's a neutral change that won't be eliminated, or even a partial color change to be light-gray helps with stealth.
For mimicry it's different, partial change is harmful. It wouldn't look anything like a target, and it would attract unneeded attention.