I live in Tucson and occasionally choose to eat at Chipotle none the less. There isn't a taco shop around that makes a veggie burro anywhere close to Chipotle's guacomole burrito. While a caloric load in itself, half a burrito is a good sized meal and is lighter than the comparison.
Is it Mexican? Not quite. Is it pretty tasty? For fast food, at least.
Chipotle is a national chain that is highly-profitable. Consider it the Starbucks of Mexican food. Not as good as the "local" place, but consistently okay everywhere you go.
They got involved because the first Chipotle was right near the McDonald's corporate offices in Denver -- the executives were eating there so much, they bought into the company!
Adding more weight to your point, I read that McDonald's divested their stake in the Chipotle chain, giving the reason that they wanted to focus on increasing revenue at their own restaurants.
Rick Bayless is based in Chicago with Frontera Grill and is considered an expert in authentic Mexican food. He has won multiple James Beard awards including national chef of the year.
"The 2000 census counted more than 530,000 Mexicans in the city of Chicago, with more than 1.1 million in the metropolitan area."
http://www.encyclopedia.chicagohistory.org/pages/824.html
There are hole-in-the-wall Mexican restaurants everywhere in Chicago.