OR...
Its because previous RAM CEO Fred Diaz was working with Cummins to get a light duty Cummins diesel in a light duty RAM (what I really wanted). But then FIAT bought Chrysler (and Ram with it) and Fiat killed the Cummins diesel in favor of their own italian diesel (which is a nice motor too).
So Fred Diaz left Ram and went to Nissan where he continued plans with Cummins on the light duty V8 diesel and thats why we will see it in a Nissan instead of a Ram.
Oddly, the next-gen Nissan will use Ram underpinnings, including the highly praised coil-spring rear suspension. Which will all make the new Titan a formidable truck ... on paper.
Some of this is not entirely accurate.
RAM and Nissan were working jointly to develop new 1/2 ton trucks with a diesel and the same chassis, different bodies/interior. That deal fell thru. Nissan continued the relationship with Cummins and developed their new Titan in-house - it will share NOTHING with RAM. RAM decided that the Cummins 5.0 would not meet their target MPG, and instead decided on the VM 3.0.
Now, if you have seen a credible report of the 2015 Diesel Titan slated to use the same underpinnings as the RAM 1500, please provide the link. I'll say again, that was the ORIGINAL plan, but that partnership fell apart some time ago (2011 I think it was).
True, Nissan needs to update the Titan's looks (and they will), but I don't consider it an ugly truck by any means. IMO, the F150 is ugly. I would agree that RAM has the "sexiest" looking truck, but looks aren't everything. The new Titan will out-perform the RAM 1500 in just about every meaningful category EXCEPT fuel mileage. And the difference will be minimal, possibly even equal or better once loaded near capacity. The RAM 1500 has a relatively low towing capacity as well (6700 lbs.), and I expect the Titan to hit close to (or over) 11,000 lbs. (current models are up to 9400 lbs. with the 5.6 gas engine).
Then, take the RAM and add lift and bigger tires, and the MPG gap between an equally-lifted/tired Titan will evaporate. Mark my words, I'll bet money on it.
Personally, I'm looking forward to the new Titan. Not sure if I will go for it, as economics need to prove it worthwhile before I'll jump (my truck is paid for, and I really want an RV). If Nissan butchers the looks, then I'll likely consider the RAM (or just go 3/4 ton RAM).