Originally Posted by
dmurphynj
United (and every other airline) have route planners and operations managers whose entire job it is to identify things such as this. There are many, many factors that go into such decisions -- way beyond anything you or I know about.
x2.
Last year I got the chance to tour UA NOC (Network Operations Center) where the planners roost and, well, plan. Quite eye-opening to see all the bits and pieces that go into making UA, or any airline, run.

UA NOC @
Willis Sears Tower
Seby12 - the other side of the coin is a slew of non-customer facing constraints the carrier may face as well. For example, an inbound 763 may fly to EWR only to be pulled out of commission for the rest of the day to tackle MX or some other issue. Or perhaps the 763 that went MX needs to get to SFO where it is currently scheduled to go into routine maintenance.