beer, parties, and a huge tourist fad.
Cinco de Mayo isn't actually the Mexican independence day, our Columbus Day is their "el día de raciónes" or Day of Races, celebrating the blending of the two races.
They just say "Wtf is Cinco de Mayo? Oh well, if you want to party down here with us, you bring the money and we'll supply the alcohol! WE LOVE TOURISTS!"
So it's all a tourist industry.