It's mostly because of the regulations involved. The U.S. has some of the most stringent emissions rules in the world when it comes to vehicles(especially diesels). As a consequence,most manufacturers can't justify selling diesel engines in passenger cars due to the certification costs and passing said costs on to consumers(so far VW and M-B are the only ones willing to push the envelope). Also,diesel fuel is more heavily taxed than gasoline is here in the U.S. so operating costs also come into play for a lot of consumers.
1975 MG MGB Mk IV "Downshift"
2012 Fiat 500 Abarth "Scorpion"
2013 Fusion SE "Epyon"
2003 Nissan Frontier SVE "Trailbreaker"
Team Duratec #241-Bailout Money?!?! WTF is that??