It is obvious that you haven't been around the industry for a long time. Take a look at some of the older photos on Airliners.net, and you may be shocked to see just how common it was for most airlines to operate big equipment into smaller cities. There are plenty of 707's, DC-8's, CV-880's, and 727's that were flown just about anywhere that had the runways to handle them. As Jim said, there were enough passengers to fill the seats at that time. In those days there was no Express flying to take traffic away from Mainline flying. Once the RJ era began, that all changed. It is common knowledge that the onset of the RJ's was nothing more than a way for the airlines to screw mainline people out of decent paying jobs from top to bottom. Today we have "Express Jets" flying around most of the system on routes that were once mainline. These so called Express flights now carry anywhere from 50-76 passengers, with their employees earning wages just above poverty level. It is just recently that Delta has started to replace some 50 seat Express flying with Mainline, and they were the ones that kicked off the RJ craze years ago.
To sum it all up for you, there were and are small cities that can support larger A/C when and if they are used. The trend has been to replace 2 mainline flights a day with 4-5 Express ones with hopes that the passengers don't notice what has taken place. Great we have more flights with less seats adding to the ATC congestion all over the country.