It's probably not that Detroit is being targeted, but Detroit does have a very large Muslim population. Perhaps the terrorists figure taking flights to a city with a large Muslim community would raise fewer red flags.
It’s also the only major airport I can name, with an approach in foreign airspace, an easy route in and out of that foreign country on the ground, and is a major population center for sympathists. And, despite the ongoing and shameful obliteration of that city, it’s highly symbolic and emotionally charged. It’s Detroit.
Not sure why that might matter here, but it could. Is there another that fits this description? El Paso, San Diego? You’re not going to cross rapidly or even relatively unimpeded into Juarez or Tijuana. Not a center of Muslim immigrants, either.