Originally Posted by ashill

I suspect it's partly because too many passengers put their bags in the first open overhead bin they see (sometimes closing the bin right away). If the first pax on board are all in the back of the plane, that means that the bins up front will be full before the passengers actually sitting there get on board. There may be some space in back, but it doesn't get used because late-boarding pax who sit up front can't find it.