I know in the case of some airlines, it's because their luggage fare structure encourages people to use carry on rather than checked luggage, which results in t

I know in the case of some airlines, it's because their luggage fare structure encourages people to use carry on rather than checked luggage, which results in the overhead space being oversubscribed. Even if each bag technically fits within the carry on requirements, there's not space for all of them. Also as an intermediate step before gate checking bags, they usually make people take backpacks etc. out of the overhead because they _can_ fit in the under seat area, even if you didn't bring any other carry on items, which also disincentivises packing light - really you want to use a hard carry on that _has_ to go in the overhead to ensure that it goes in the overhead rather than be forced to give up some of your already limited legroom.