Because it's a race for male professional cyclists? There's version of the TdF for women, too. I know that may not seem like much of an answer, but it's like a lot of professional sports. There are women's teams and men's teams and sanctioned events that are specific to each.
But I'm no Tour expert or even follower; perhaps someone has a more official sounding answer than that.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher