I don't know for sure, but I do have some theories. One is the obvious reason, cosmetics. You know "Chevy orange", "Ford blue" etc. It may be that since diesel eng. blocks are cast iron, they paint them to keep that "new" look, instead of the rust look. Personally, I like the industrial machine look of my engine and the related components.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section