I agree mainly 3 reasons.
1. Emissions
2. Clutch (GM just fails miserably at this from clutch pedal assembly squeak to flywheel and disc hub etc).
3. Perception and Market.

3. GM has literally STUNK at clutches since well I am not sure they ever made a nice clutch. So most GM loyalists have given up and converted to Auto's

In comparison Ford and Dodge seem better easier clutch pedal pressure and smooth quiet motion. Yeah they all tear up if you up HP and push the limits but Manuals with good clutches can be mated to high HP. Auto will still take a bit more if also built in the light trucks but again GM could do so much better....

I am hoping they will eventually try again but in 10 more yrs a manual will be a collectable.