I think DIA is the biggest bust there is, stapleton had a future and room for expansion, and they needed to do some remodeling but that was it. unlike DIA where there is no room for expansion. plus gatespace is at a true premium, everygate is occupied so that makes a difference too. and most foreign airlines find it easier to codeshare on the flights by connecting to AA,DL,AC,UA,NW,CO in MSP,DFW,ORD,DTW,IAH,PHX,JFK,EWR,LGA,YYZ,YVR,LAX,SFO,ATL,IAD and other us airports where as an example,JAL doesn't serve DEN but i am pretty sure that it codeshares with AA. and other airlines think it's better than to waste a slot for instance,DEN-IAD-CDG
instead of UA wasting a slot, they can combine DEN with another airport and both of the airports can combine to get the load factor needed for that flight.