The purchase price when buying a diesel, higher fuel costs, higher maintenance costs, higher repair costs, can make the diesel not worth the money.
Driving a 20 mile route 5 days a week is 100 miles. 30 weeks of mowing and that is 3,000 miles.
Not a lot of fuel saved. along with the higher purchse price and fuel prices, people may never reach the break even point where the they save money buying a diesel.
Then there is more engine noise, diesel smell.
Most LCO's I see are not pulling 2,000 lbs on their trailer.
As pointed out 500,000 miles are pointless when the frame, body, fuel, brake lines, are all rusted away before diesel engine can go that far.
An 18 wheeler is going to put on an easy 130,000 miles a year and can move 80,000 lbs.
I think many people buy diesel trucks without doing all of the math to see where the break even point for them will be.
Basing a decision to buy a diesel because it is 3 seconds faster to hit 55 mph. Or a diesel can hold a grade longer before it down shifts. These factors provide braging rights, but they do not point out where the break even point is.
I googled and found that the break even point can be up to 10 years.