For one thing, they drive on the wrong side of the road and Detroit doesn't design their cars to have steering on either side. So that keeps out the vast majority of cars.
That's actually trivial. In a lot of plants around the world, LH and RH drive cars come off the same line.
The problem isn't simply not having small-car designs either: they do - in Europe (the Ford Fiesta is simply the first one that comes to mind).
The US carmakers chose to neglect the economy segment - and now they're paying for it. The fact is that what we're experiencing right now is a top-down disaster. -Paul Krugman