I recommend novelist Oscar Casares' take on Cinco de Mayo in the Houston Chronicle.
The holiday, which has never really been much of one in Mexico, crossed over to this side of the border in the 1950s and 1960s, as civil rights activists were attempting to build harmony between the two countries and cultures. The date gained more attention in the 1980s when marketers, particularly beer companies, saw this as a perfect opportunity to capitalize on the celebratory nature of the holiday.
Also important for marketing:
Of course, if you happen to not speak Spanish, Cinco de Mayo is much easier to pronounce, no matter of how many margaritas are involved. So a facility with the language and how this lends itself to marketing products around the holiday certainly must also play a role.
Just imagine a beer company trying to fit Dieciséis de Septiembre on a beer koozie.