Maxalt,
ORD was formerly a slot airport until John McCain (among others) lobbied on behalf of the low cost folks that the slot system (the "portfolio" of mainline carriers) unfairly disadvantaged the customer trying to fly for tuppence ha'penny. Result...grid lock (but cheap fares).
Slots were lifted and LGA and ORD, to name a few, became parking lots at certain times of the day. As you may be aware, ORD's layout also complicates departures/arrivals when the wind blows from certain directions.
Not surprisingly, Johnnie Mac has not been heard from lately in this area-no doubt he's probably busy talking up the next issue, to increase retirement above age 60.
Also to be fair, the major airlines have introduced RJ's on many routes with the result that two groups of fifty people takes up more "room" than one group of one hundred. The mantra amongst many airlines has been frequency of service, ie leave when the business folks want to every 45 minutes or so, rather than go in a larger airplane every ninety mins (simplified but you get the pic).
One last issue is that the FAA considered reduced spacing on final. I believe ATC/airlines are pushing back on this as a study revealed an increase of only four or five arrivals creating extra ATC work and clogged taxiways (as there is nowhere to park the additional volume-it's bad enough as it is!).
The simplest "cure" is to charge much higher landing fees for RJ's. Airlines would be more likely to use their larger equipment into ORD and have one mainline airplane carry two RJ's worth at peak times.
dd