Romeo and Juliet- nurse and mercutio speech

The character of Mercutio is a charismatic and charming one. He is the most articulate character in the play and the movie, and he is a brilliant wordsmith.
On the way to the Capulet ball Romeo says of Mercutio: ‘You have dancing shoes with nimble soles’ (A-1, S-4, L-13-14). The significance is that Mercutio is literally a dancer, and a flamboyant fun-lover, as we notice in the movie at the Capulet party, and metaphorically a dancer in his skillful use of words.
Mercutio is a character that likes being the center of attention, Romeo describes him to the Nurse as: ‘a gentleman… who likes to hear himself talk.’ (A-2, S-4, L-144-145).
Mercutio is also the character that knows Romeo, he can and does, criticise’s Romeo’s fickleness in love. He can do this because he and Romeo are like a pair, like Romeo and Juliet.
In the movie, Luhrmann makes the decision to play Mercutio as black, gay and a cross-dresser.
But why portray him like this?
I believe Luhrmann made this c...