Well, some feel that they can improve the older diesels to perform similar to the newer diesels. They also feel that they don't want to have to deal with the newer emissions system, especially if they are out of warranty. The older diesel aftermarket is also really mature and repair parts can be bought relatively cheaply should something break. Also, since they don't have all the emissions, they tend to get better fuel economy than the 2007 to 2010 emission vehicles. But the 2011 and up vehicles were back to matching fuel economy with the roll-out of DEF (Diesel Exhaust Fluid). Also, the older diesels were more tolerant of impurities in fuel. Heck, some people would just pour their used motor or transmission oil through a coffee filter into the tank and run it through the motor (I personally don't think you should do this, something about the potential of metal shavings really doesn't sit well with me).
Newer diesels make more power and run much cleaner than the older vehicles. But along with all that power and cleanliness, is the complexity of the emissions system. Some don't feel like messing around with the new components and some are mortified at the prospect of pricey repairs should things break (New Ford 6.7 is roughly 13k). But that's what I feel a warranty is for and by the time these vehicles are older, there should be a mature aftermarket for them making repairs somewhat cheaper.
I think that sums the generalities of why some prefer older over newer and some contrast with a newer vehicle.
