Chipotles? Guajillos? Your fundamental knowledge of Mexican cuisine makes me think you've never been south of the border. I'm not dismissing Tex-Mex's world influence, but calling it the BEST MEXICAN FOOD is patently ridiculous. Especially to people living in Paris who have to eat that greasy kidney bean & ground beef dogfood served up as Tex-Mex in most of Europe.
That being said, I encourage you to check out the writings of Gustavo Arellano, who is the foremost English-language food writer specializing in the ethnographic history of Mexican food in America. Specifically this book.
All is explained.
If you're interested in becoming more of a burrito scholar yourself, I highly recommend this book.