Cumin in Taco Seasoning?

I'm going to supplement Cindy's answer, by addressing this part of the question:

When and why did cumin get associated with tacos?

According to Wikipedia, Serious Eats, and History.com, the cumin was brought by workers the Spanish imported to Texas from the Canary Islands in the 1500s, who themselves were the descendants of North Africans and hence had a love for cumin. These workers introduced cumin into Tex-Mex cuisine, particularly Chile Con Carne, and when Willie Gebhardt created the first bottled "chili seasoning", it included cumin. Many Americans encountered Gebhardt's seasoning a generation before they experienced any other kind of Mexican food, forever associating it with the cuisine. And, for that matter, used Gebhardt's for taco meat, as my Oklahoma branch of the family did back into the 1930's.

The reason you don't encounter it in some Mexican restaurants it that the culinary staff are from Mexico, and not Texas or near Texas. As such, they never "picked up" cumin.

(and yes, this does mean that Thrillist is wrong about the origin. India was not involved)

As per your comment, the reason is Tex-mex flavor. Most of the pre-made 'Mexican' seasonings we get in the US have cumin and probably various other spices or flavorings not associated with authentic Mexican dishes.

If you look at recipes for authentic Mexican dishes, you won't typically find cumin in the ingredient list. While I like cumin in some Indian dishes or in chili, I find that I don't care for it in Mexican dishes. (And to be honest, I find it just as easy to add any desired spices as it would be to use a prepared package.)

You may find this article on Thrillist about the difference in Tex-Mex and Mexican food interesting. I'm sure an Internet search may yield many other articles.

Edited to add more info. From the linked article:

The differences between Mexican and Tex-Mex food can be summed up in the use of a few key ingredients found in the US that are scarcely used anywhere South of the Rio Grande. These ingredients are: beef, yellow cheese (like cheddar), wheat flour, black beans, canned vegetables (especially tomatoes), and cumin.

Chances are, if you're eating anything with one or more of those ingredients, it's Tex-Mex. Beef was the meat of choice for Texan ranchers back in the day, but it's hardly used in Mexican cuisine outside of the extreme Northern reaches of the country.

Cumin was imported to the US and England from India, and, while it's been slowly incorporated into dishes in Mexico, the US was quicker to adopt it as a spice. And let's face it -- gringos love wheat, and pretty much any burrito you'll find up here is wrapped in a wheat tortilla rather than the maize-based tortillas down South. A general rule is that the more starch there is, the more Tex-Mex it is (with the notable exception of tortas).

I would like to point out something in addition to Cindy's answer which I do not see addressed, as I feel it adds much to the reason of why as well: The predominant flavors of most authentic Mexican tacos are very simple, although delicious: Protein, Cilantro, Onions, and Corn Tortilla (perhaps Lime, depends on where you are). The blend of these flavors is very much a great combination, and needs little else, as the tortillas, cilantro, and onion all have a very pronounced flavor, even if the meat is more bland in nature.

Of course, from my understanding, things are a bit different depending on region as well, as tastes and common foods change as they would anywhere else. In truth, there is a good bit of variance between TexMex, "Mexican", and truly authentic Mexican (both in restaurants and at home.)