GM claims low sales, and while that's true, there are reasons for that beyond what the market demands.
CAFE - part of the calculations are based on vehicle footprint. The same 5.3L engine is a way worse "hit" in a small RCSB than a larger CCSB. That give GM corporate incentive to sell fewer of them.
Markups - I bet it's $1-2000 difference in their cost to build them with the extra doors and seats. But they can sell them for a much higher premium than that. So dealerships are incentivized to sell more of those than the smaller trucks.
Rebates - When I bought by RCSB, they barely budged off the price. I paid $42k (Canadian). I tried and tried, and all they kept doing was offering me a similarly equipped double cab for less money. How does that make sense? From a purely financial perspective I was stupid not to take one of those other offers. However I don't want a big truck.
Trims - if you want the higher trims or nicer options, they force you into a 4-door model. When I bought in 2014, LT Z71 was the highest model you could get. No leather, no touch screen (I think that changed later on), etc. The majority of the two-door trucks of either box length they made were optioned to be cheap work trucks. The 6.2 was unavailable in a regular cab at all.
If GM offered regular cabs at proportional prices to the bigger trucks, with all options available, and similar rebates, they would sell WAY more than the pitiful numbers they point to as justification.
As it stands, THEY STILL BUILD THEM. They are not saving money at all since they still make them for Mexico and the middle east. So the "it costs too much to make small production numbers" is bogus.
Anyway, I own my 2014 RCSB, and I guess that will be it for me buying new GM trucks. I'll just have to keep mine on the road as long as possible. That was kind of the plan already, but I did plan on buying a new one eventually.