With a large Mexican population here, it makes sence in a marketing point of view. The same as the Irish and irish culture and Saint Patrick's Day.

I also found this more lengthy explanation...

In 1862, at the time the Battle of Puebla took place, the United States was engaged in its Civil War. The French presence in Mexico was a strategic move: by gaining a toehold in Mexico, the French could then support the Confederate Army. The defeat of the French at the Battle of Puebla was not definitive, but it helped to stave off the French while the U.S. Union forces made advances.

Thus Cinco de Mayo can be seen as a turning point in the U.S. Civil War. Cinco de Mayo was first celebrated in the United States in Southern California in 1863 as a show of solidarity with Mexico against French rule.

Celebrations continued on a yearly basis, and by the 1930s it was seen as an opportunity to celebrate Mexican identity, promote ethnic consciousness and build community solidarity.

In the 1950s and 60s Mexican-American youths appropriated the holiday and it gained a bi-national flavor, and its celebration was used as a way to build Mexican-American pride. Celebrations sometimes acquired corporate sponsors, and this is the way the holiday began to take on a commercial flavor.