I'll caveat off of what Exedra said...

Once flying from Frankfurt stateside, we had to turn around and go back for a mechanical. Once fixed, we had to wait over three hours for a takeoff slot (must have been the busy season). Super-airports like that have almost no margin for error or change of plans.

Again, I sympathize, but imagine the nightmare the airline has with 300+ pax now needing either new flights, hotels or other considerations due to one person's medical condition because they had to give up a takeoff slot. The US bound flights don't have a lot of flexibility, and the intra-US flights often don't, either.

And, as was posted before, the weight and balance issue with some small planes is certainly a factor on take off. Too much weight up front, and the plane may as well drive to the destination airport, because it ain't getting airborne.