When I lived in Western Kansas the elevation was around 2100 feet, probably a bit higher. That is as high as some of the smaller mountains in the East.
I bet the Eastern Colorado plains is a bit higher.
Sounds as if the same hydration of the lower crust, making it lighter might well be at least one cause.
CO’s low point is ~3,325’, where the Arikaree River flows into KS near the CO/KS/NB tri-point.
Woops...I meant to say NE for Nebraska, not NB. My bad.