Because, given the demonstrated ways racism "works" (as in, how it's practiced, and what effects it has when practiced between differentially enfranchised groups) it's not accurate?
Would you mind clarifying the distinction between "differentially enfranchised groups" and "race == power" ?
One describes a tendency; the other asserts an identity relationship.
It doesn't map perfectly, but it's close enough for the bandwidth I have to keep contributing to this thread right now.