A Colorado Crew Cab 4x4 LT starts at $34,500, so if the diesel is a $4k bump, that puts the cheapest version at $38,500 before freight and taxes. Yes, the LT is "only" a $3400 bump over the WT, but that is cash that some would want to spend on the motor - not on power seats, alloy wheels, and other bling.
Meanwhile, Ram will sell you a 1500 ST Tradesman Quad Cab 4x4 with an Ecodiesel option. Yes, you really need to add a few items (LSD, ST Popular Equipment Group, Tow Pacakage) to make it livable. Price? Around $38k, also before freight and taxes.
I'm not implying that full size trucks are inherently better, just that Ram has decided to give buyers of even the most basic truck access to the most work ready engine.
Just imagine how awesome a work truck a Colorado WT with this diesel be? Why does GM do this? First the Cruze Diesel, now this. What would the cost be to GM to have this engine be an option across the board???