Part of the problem is in the growing use of electronic check-in. When you check in at a machine, or from home, there's no check-in agent to eye up the size of your bag. So you go through security (some places enforce limits at security, but that seems pretty rare), get to the gate... And, of course, not all gates have easy luggage check facilities. So many airlines say "ach, well, let 'em fight it out". Some may start to check large bags and trundlies if it's a full flight. But 9 times out of 10, they plump for the general melee...