I understand that on average there are differences in eye structure that make women better at color differentiation and men better at hand eye coordination, but I've never checked with a real source on that
Almost certainly, the bigger factor is that the league darts, is totally dominated by large groups of partially-inebriated, sometimes hostile, men. A woman who sets off to become good enough at darts to play and win at the tournament level has to spend time in league darts learning strategy. During her time in league dart she will spend some time beating guys who react poorly to the thought of "losing to a girl".
Simple solution: Rainbow Darts! Gender-balanced, prettier, and adds LGBT representation right into the game!