I think that this could be a little misleading to a certain extent. I think that there are a couple of reasons why Ford sold more diesels than the competition, and it's not necessarily because they are a better diesel. First off is that for a number of years Ford had the only truck on the market that had enough front axle capacity to allow a diesel engine and a 9 foot plow. So if you wanted a 9 foot plow AND a diesel your only manufacturer recommended option was Ford.
Secondly, and I think perhaps even more importantly is the fact that GM and Dodge had better non diesel engine options on their heavy duty trucks. While I'll admit that the V10 in the Ford was a great engine, a lot of people were scared (probably unnecessarily) about feeding 10 cylinders worth of fuel. The 5.4 V8 is a great half ton truck engine, but it's a lousy 3/4 and 1 ton truck engine, especially when you've got the heaviest rolling chassis on the market. I'm not saying that the 6.0 V8 in the GM's or the 5.7 Hemi are awesome in heavy duty pickups but they are in my opinion at least adequate whereas the 5.4 is not, and I say this as someone who has spent a lot of time driving each of those gas motors. So when you consider that Ford didn't even make a lot of V10's and the 5.4 is pretty underpowered in the Super Duties (especially the 2v motors) then you're pretty much left with owning a diesel if you want to get a Super Duty, which unquestionably led the pack in heavy duty pickups for a long time.
Lastly I would note that GM and Dodge 3/4 and 1 ton trucks were styled very similarly to their half ton counterparts, whereas the Ford Super Duty is styled completely differently than the F150. I know of people that hated the look of the F150 but loved the look of the Super Duty so they bought Super Duties based upon looks alone, even though they didn't need more than a half ton pickup. So when you add all that up, I realize that Ford still outsold GM and Chrysler in the 3/4 and 1 ton truck market, and I would add for good reason as they had some serious advantages that GM and Ram have only recently caught up to. However, I would argue that GM and Chrysler had better non diesel engines available in their 3/4 and 1 ton pickups than Ford did so a lot of their 3/4 and 1 ton truck buyers were able to get by without ponying up the bucks for a diesel.
I say all of this as a current owner of a 2011 GM 2500HD gas truck AND a 1997 Ford F350 7.3 Powerstroke. I see the value in all of the trucks and spend my money on what turns out to be the best value for me at the time. Oh and even though I only had it a short time my 6.4 Powerstroke equipped F250 never gave me any problems and was a powerhouse that pulled like a freight train. I liked it quite a bit.
