Diesels have flourished in Europe because of expensive gasoline and tax breaks for diesel.  Many European governments promote diesel because it emits less carbon dioxide into the atmosphere.  On top of lower fuel prices, diesel drivers experience increased fuel efficiency.  The diesel engine is a more efficient design than the gasoline powered engine.  

So, what’s stopping diesel engines from invading our cars here in America?  It may still happen but there are a couple of obstacles.  

First, diesel fuel is not cheaper than gas here in the States.  We enjoy lower gas prices than our European friends, and our government does not offer the nice tax breaks on diesel like most European governments.  Therefore, diesel is harder to come by and more expensive in the U.S.  
   
The main reason for the tax breaks in Europe and lack of them here is a difference in the way we set up environmental standards for our vehicles.  This is the second obstacle.  In Europe the focus is on low carbon dioxide emissions but here in the States the EPA wants low levels of nitrogen oxides and particulates.  This is because nitrogen oxides may be carcinogenic and particulates help form smog.