Well depending on who you ask, a bit of both. A lot of manufacturers have not wanted to put a diesel to market that they couldn't sell in all 50 states and with California's strict emissions standards, a lot of diesels couldn't meet the standard. Diesels in general spew high levels of nitrogen oxides, gases believed by some experts to be carcinogenic, as well as soot (or particulates), which contribute to the formation of smog. This has been a focus of the EPA, whereas European pollution standards focus primarily on carbon dioxide emissions and so are achievable by diesel cars
There have been plenty of excuses brought to bear though through the years. Americans had such cheap gasoline, there is no reason to drive a "stinky" oil burner. People remember the botched GM diesel of the late 70's early 80's (a real abortion -- gasoline engine converted to diesel).. And all the stereo types they have against diesel engines (hard to start in cold [not true], really noisy [though slightly noisier than a gasoline engine -- noisy is not true]...
Next, it was manufacturers were waiting for the switch to ULSD before introducing new diesels to market.
Now, with the rising cost of fuels and the availability of biodiesel, there is a lot more interest in diesels again. This is especially true with truck/suv enthusiasts. It doesn't matter which forum you go in, be it Toyota or Jeep or Nissan, everybody wants a diesel, but the manufacturers almost refuse to recognize the demand, all the while producing the same vehicles either for export or for foreign markets. Jeep has teased us a bit, but that is the only one to take the leap.
