My theory is that it goes back to the days of the Rule of the Detroit Dinosaurs, when they were marketing the slushboxes as higher end and luxurious. That sediment has just stuck. Same reason why cars for the US market often feature softer suspensions than their rest-of-the-world counterparts.
Mercedes, and to some extent BMW, are still guilty of this, Merc basically doesn't sell stick shifts in the states because they feel that it would somehow tarnish their image as a "luxury" car.
Go figure.