Nine weeks ago I set out on a mission. I had just arrived in Mexico for a journalism internship in San Miguel de Allende, and was trying to figure out why more foreigners — especially Americans — don’t travel to Central Mexico, and if they should. Americans make up the majority of international arrivals into Mexico, but most head for the pristine coastline in Cancún, Cabo and Puerto Vallarta. Very few head to the country’s high-elevation center…