I've always been skeptical of this. Not whether it's true; obviously they don't sell as well, but why? And when did it start?
Regular cabs have been overwhelmingly stripper-spec forever (7/109 single-cab F150s here are non-XL, for example), so it's not hard to imagine someone shows up wanting one and then goes "wait a minute, only crew-cabs have the package/feature/trim I want? Well then."
Did automakers introduce crew cabs, all other things being equal, and sales suddenly blew regular cabs out of the water?
Without sales figures over time broken down by cab type--which aren't released--it's always going to feel like a chicken-or-the-egg problem: did people stop buying regular cabs on their own, or did companies start making them mediocre?