For the record: Fajitas is not a Mexican dish. It is either a Tejano dish or a Tex-Mex dish, depending on whom you ask, but it is certainly not a Mexican dish.

Furthermore, Fajitas are not chicken or shrimp or vegetarian or anything other than BEEF. Fajitas are made from a cut of beef, specifically, skirt steak, which is similar to flank steak, but not the same. They came to popularity originally among those of lower economic means as they were cooked using a relatively inexpensive cut of meat which was, at the time, cheaper than other cuts of meat. As the fates would have it, with the increased popularity of fajitas, the price of the cut of beef has risen dramatically, making what was a traditionally affordable dish now quite expensive.

In sum: Not a mexican dish, not a style of cooking, made from beef. Now, will the real fajitas please stand up?

Added about 5 mins after original....
And another thing. Sour cream is a largely unknown ingredient in Mexican food. What is used in Mexican cuisine is just called crema, and it is vaguely similar to whipping cream, or perhaps to creme fraiche.