Glossy white plastic hides scratches and fingerprints better than glossy black plastic.

From a purely practical standpoint, white is a much friendlier color for daily wear and tear. Glossy black plastics show tiny scratches, dust, and oily fingerprints almost immediately because of the way light bounces off them. White plastic scatters light in many directions. This scattering hides minor scuffs and daily dirt. Because AirPods live in your pockets, bags, and ears, they get beat up quickly. White keeps them looking clean and new for much longer than a dark, glossy material would.