Occam's Razor, I suppose. Something roughly like emotivism seems like a wholly adequate explanation of what moral judgements are, both from a psychological and evolutionary point of view. I just don't see any need to presume that moral judgements would be anything else, nor do I know what else they could be. From a decision-theoretical perspective, too, preferences (in the form of utility functions) are merely what the organism wants, and are simply taken as givens.

On the other hand, empirical judgements clearly do need to be evaluated for their correctness, if they are to be useful in achieving an organism's preferences and/or survival.