Because there is no better place to store eight pounds of guacamole than in an authentic Mexican hat. Or perhaps you are auditioning for Martin Short's role in your community theatre troupe's production of The Three Amigos. Then again, why can't it be both?
Because your internet girlfriend left you for a man completely made out of rock hard abs which led you to nothing more rational than to dress up as a mexican bandit, robbing highway travellers to support yourself, while riding velociraptors in order to make it to the city where ab-man lives and kill him. (too obscure? check out my reference: http://www.drmcninja.com/page.php?pageNum=26&issue=3)
The winner is kingcasey for his answer "because you look good in stereotype". Missnikki is the runner-up.
Sean gets the loser award for not even trying.
The real answer was because I look good in stereotype. Also, I was attending a costume ball, and had recently bought a sombrero (for $1 at a garage sale).