Quoting from further down the Wikipedia article:
"The holiday, which has been celebrated in California continuously since 1863, is virtually ignored in Mexico."
"Cinco de Mayo started to come into vogue in 1940s America during the rise of the Chicano movement."
"The holiday crossed over into the United States in the 1950s and 1960s but didn't gain popularity until the 1980s when marketers, especially beer companies, capitalized on the celebratory nature of the day and began to promote it."
There you go, beer companies are to blame for the american magnification of the mexican holiday.
So, being mexican and all, I also vote for 5 de mayo. My main reason is IBRD is a global event and 5 de mayo is not a general celebration, even though it is of mexican origin it is not largely celebrated but in 1 mexican state (out of 31), and is not even widespread in the US, as only some states know about it. We shouldn't displace a global event in favor of a very localized "holiday".