In addition to everything that has already been said, the heavier a bag is, the more likely it is to fall out of a passenger's hands when s/he is trying to get it into or out of the overhead bin and hit another passenger. And for carriers like QR on which cabin crew actually assist with overhead bags (unlike most domestic U.S. carriers, where cabin crew have a clause in their contract which says that they are not required to assist passengers with carryon bags), there's the possibility of injury to cabin crew if the bag is too heavy.