It is an apples to oranges comparison because one was open and the other was not. Also, it was not picked up on just how Republican Indies were going to vote and knowing that result would have made IA more likely but IA indies are more elastic I suspect then Colorado's where more of them are "too leftwing for the Democrats" Indies in Boulder.
CO and IA used to always vote together. 32 (FDR), 36 (FDR) 40 (Wilkie) 44 (Dewey) 48 (Truman), 52 (Ike) 56 (Ike) 60 (Nixon), 64 (LBJ) 68 (Nixon) 72 (Nixon), 76 (Ford), 80 (Reagan) 84 (Reagan). Only 1988 does IA diverge and start vote for Dukakis because of the previously mentioned farming crisis.
It is always one of the most undiverse states in the union, whereas CO has a strong minority voting bloc. The Republicans therefore between the more friendly indies and fewer minorities have a higher ceiling then Republicans in Colorado.
Having more minorities, especially Hispanics and Asians, in a midterm election is not advantageous for the Democrats. Look at the results from Adams, Pueblo and even Arapahoe to an extent. The Democrats didn't have that much trouble from the suburban white independent vote in Colorado, it was the Hispanics and rural/exurban independents who swung it for Gardner. Looking at the results from Broomfield and Jefferson, one would think there was a recount going on right now in Colorado.