Cost is high
The EPA has more stringent requirements than Europe does for controlling NOx – Nitric Oxide, the key compound that produces smog. The technology to meet these standards exists, but it’s expensive (to develop, manufacture and certify), which is why many of the diesels you see on the market in the U.S. are at the high end of the price range.
This issue is also the reason why there are so many diesel cars available in other countries that are not available in the United States. Simply put, it’s so expensive to develop a diesel engine that meets US emissions standards that most manufacturers don’t bother. Particularly since diesels have never been a popular engine choice in the US market.