Why do Diesel engines get so grungy and messy?

[Carrameow]

I finished putting back my 85 300D Diesel engine back in my car this weekend (My indy had plucked the engine from an old lady's car with 160K that he serviced that had been hit in the rear and front.) But not before spending two days chipping and scraping and spraying degreaser over it to get some twenty years and Thirty pounds of sludge off of it.
It dawned on me that when I changed the engine on an 83 Volvo there wasnt even as much as 20 % of this Cr_p.
The worst area is always the big oil pan where crud esconses itself in the 2 sq inch grids of the lattice work of the casting where it meets the transmission. Another place is the crank pulley, which I actually removed to get to the sludge.
Another place is the interior side of the injection pump and of course the engine face. I wanted my engine to look New, like it came off of the assembly line.
It gleamed like aluminum and black jewelry when I was done. I woke up and had my coffee and actually went out to stare at it in disbelief. I dont know how long it will last.