You're making plenty of unfounded assumptions. For one, brand, pickups are bought on brand above all, why do you think people buy Powerstroke Fords when they should know better?
Two, its not about capability except for a small subset of the population. Nissan dealers are concentrated in urban areas which is great but they cater to a fuel efficient mindset, a 5.0L v8 Diesel is not exactly a frugal motor. The guys who do demand capability live in the rural areas that the Big 3 service almost exclusively...
Having worked with and on the 7.3L, 6.0L and 6.4L Powerstrokes, they are actually really well engineered engines. The 7.3L is in my opinion one of the best engines put into a work truck. It doesn't have too much power, it has plenty of torque and has a reputation of being a long lasting engine when maintained properly.
The 6.0L diesel is one of the most enjoyable engines I have ever driven. It is slow to respond but makes all the right noises and pulls hard under boost. Properly maintained the 6.0L will last a long time. The biggest flaw with this engine was emissions equipment and inadequate maintenance. Unmistakable, unique exhaust note that sends shivers down my spine.
The 6.4L pulled harder in lower rpms but really ran out of steam near redline, it was not as enjoyable to drive. I was never inside a 6.4L, but from what I've seen it shares the design logic of the 6.0L.
I found the 6.6L Duramax to drive very much like what I think the 7.3L PSD would feel like if it were modernized. I find myself frequently mistaking the exhaust note of duramax trucks for 7.3Ls.
The 5.9L Cummins, at least the old mechanical ones are a riot to drive. Never had to do more than maintenance work to mine, didn't give me a reason to go deeper than taking the timing cover off for the killer dowel pin fix.
The ISV is going up against a history of proven diesel V8's, in a pickup with something to prove. My bet is the new Titan is going to be a rocket ship with the most refined diesel engine ever offered in a pickup truck.
