Aside from the PAWOB problem that everyone is aware of (and money/equip won't help the weight restricted RJ flights), the problem really is multifaceted. A carry-on policy that works 95% of the time doesn't work that last 5% of the time, but a policy that would work 100% of the time would be too restrictive (and non-competitive) 95% of the time.
The biggest factors that make the policy unworkable that 5% of the time are:
1 - too many rollaboards. There just isn't enough overhead space if too many people have carry-ons that only fit in the overhead bins.
2 - lack of space for crew bags. At least on the 737, 2 of the F/A's have no where to stow their rollaboard except the overhead compartments. Some pilots use rollaboards that are too big for the space provided in the cockpit, meaning another crew bag taking up space in the overheads.
3 - too many people don't use the underseat space for smaller carry-ons. With the limited leg room in coach, I honestly can't blame them but it fills up the overheads that much quicker.
4 - bringing on too many items. As has been mentioned, some folks just try to "sneak" too many things on board.
Jim
ps - a related, though different, problem. I'm seeing more folks dropping their bag at the bottom of the jetway without a gate check tag. They think it will be brought back up to the jetway at the destination, just like the RJ's.