Good question
Kelly Arthur Garrett, the Mexico City Herald’s political writer, also is their book reviewer. He reviews a non-academic history of Cinco de Mayo and the Battle of Puebla (Cinco de Mayo by Donald Evans), and asks…
How is it that Cinco de Mayo, an entirely home-grown Mexican holiday, is celebrated more broadly and intensely in the United States than in the nation that created it? …Cinco de Mayo observances in the United States have served various agendas— a recognition of the Mexican heritage of certain regions, a rallying point for Chicano pride and political activism, a stimulus for community self-determination, a pan-ethnic celebration of all things Latino, a commitment to anti-imperialism, a marketing bonanza for the commercial sector, and an excuse to party.In Mexico, on the other hand, Cinco de Mayo naturally competes with a lengthy line-up of patriotic and ecclesiastic holidays, settling in somewhere near the middle of the pack.
…
It’s useful, for example, to remind us that what we take for granted today — that the French eventually retired from Mexico, that the North eventually won the U.S. Civil War — was anybody’s guess as events unfolded in 1862.
It’s also thought-provoking, if not entirely convincing, to suggest that had the Mexicans not won the Battle of Puebla, which delayed the French advance, “the United States would never become the significant world power that it is today” and “the nation we know as Mexico would probably still be dominated by France.”
But this effort to make Mexican history “relevant” by connecting it to U.S. concerns — rather than recognizing its value in its own right — caters to the stereotype of U.S. readers’ attention being limited by their own borders.
That the stereotype is probably true (as it is for residents of most countries), doesn’t justify feeding it.
Which doesn’t mean I’m not gonna enjoy Alpine’s Cinco de Mayo party.
mexfiles.net 
That confused me when I moved from Laredo to Austin…what was the big deal about Cinco de Mayo? September 16 was the big Mexican holiday as far as I was concerned.
As if we need an excuse for getting our vitamin T (tacos, tortillas, tamales…). A friend of mine said he probably wasn’t the only guy who worked in Chicago who had fond memories of playing Benito Juarez in a grade school play on Cinco de Mayo — which is about the extent of any celebrations.