Hand baggage is not so much of a weight problem but a space problem. With low cost and charter carriers packing in the seating, there is just not enough bin capacity for everyone to bring a sizeable bag. The crew stow their wheelies in the bins alonside much of the safety equipment, so a sensible limit has to be imposed. Ryanair are trying to reduce the amount of cabin bags with new rules as they say that their loadfactors have increased recently (and most people are trying to avoid baggage charges and waiting at the other end).