They couldn't sell them. Probably because they are so ugly. They are the ugliest truck out there. I have no doubt the Diesel engine was magnificent in them. But looks matter.
Look at their sales numbers: Pretty shitty, eh? I'm surprised they haven't discontinued the Titan altogether, but maybe we'll see them do so in the next year or two.
Calendar year
United States
2010
23,416[20]
2011
21,994[20]
2012
21,576[21]
2013
15,691[21]
2014
12,527[22]
2015
12,140[22]
2016
21,880[23]
2017
52,924[23]
2018
50,549[24]
2019
31,514[24]
https://en.wikipedia.org/wiki/Nissan_Titan