Not quite true. Cummins in the last years (and I am sure others) have become DOC/SCR compliant. The SCR (Selective Catalytic Reduction) uses urea (ammonia) which combines with NO2 after passing through a DOC (Diesel Oxidation Catalyst). It practically eliminates NOx, producing carbon dioxide and water in the process. The major drawback is price and complexity.There's always trade-offs, and in this case it's pollution. Recent developments have mostly brought diesel particulates under control, but NOx has still got everyone stumped.
This video is a humerous rendition of "how easy" it is.