One other reason why the small diesels in 1/2-tons are so unpopular - cost. They're not $10,000 options like on 3/4-ton + trucks, but they are priced as premium engine options comparable to the 3.5TT ecoboost, GM 6.2, and well above Ram's 5.7 Hemi. If you look at the vast majority of trucks on the lot (back when there were vast amounts of trucks on lots...) they're optioned with a volume engine that costs ~$1,000 like the 2.7TT ecoboost, GM 5.3, or Ram 5.7 Hemi. For most folks who aren't buying or leasing a truly loaded truck (High Country, Denali, Platinum, Laramie Longhorn) the extra $2,000-$3,000 is better spent on luxury options that they touch like leather, heated seats and steering wheel, premium sound, and advanced safety systems and/or lighting. Relatively few buyers are picky about the drivetrain options, as long as it's quiet and feels powerful, they're interested in appearances inside and out - how it will impress their friends and colleagues. It's the same reason you see so many "luxury" cars and crossovers with the smaller standard engine, and often without AWD when available.
In the overland world, believe it or not, many of us are cost-conscious and purchase vehicles that we can get a deal on that have the hardware we need. It's hard to get a deal on a vehicle that's relatively rare on dealer lots, and if it's uncommon when new, it will be uncommon used also.