It's leftover from when all mammals were nocturnal. Color vision is useless at night, there aren't enough photons to care about their frequency, so mammals lost it almost entirely and only primates have re-evolved the red-green distinction. Birds were big stomping dinosaurs, and out during the day, so their color vision is great.

Some mammals can see UV even though they can't differentiate it from other blues. One theory I recall reading is that detecting UV would interfere with our unusually high visual acuity, but I forget the argument why.