Probably because most diesel owners have historically bought diesels with the intent to keep them more than five years, and many, if not most, do so in practice.
I had my 95 for 10 years and would have kept much longer if Chrysler hadn't had such great incentives at that time. In addition NADA had a high value on the 95 Dodge/Cummins 4x4 QC, which I got when I sold it.