Why are there no black or asian riders on the Tour de France?

I know, I know (I'm playing devil's advocate here), but why can't there be parallel competitions within the same event - as happens in Wimbledon, for example. Is there any reason (apart from Tradition) why women and men couldn't ride in the same race, and be awarded yellow, green, polka dot and white jerseys of their own?

(Perhaps this is a discussion for another thread...)

There is La Grande Boucle Feminine, and the women's Giro.