Someone had mentioned the low RPMs, and I believe that along with the closer tolerances of a low RPM engine plus long-wearing materials will make a well-designed diesel engine last a long time.

Check out the following links on engine life:

http://www.metricmechanic.com/pg78.htm
http://www.metricmechanic.com/pg79a.htm
http://www.metricmechanic.com/pg80.htm

Enjoy,

:-) neil