This isn't real analysis. However:

The problem with checked luggage first arises if somebody doesn't get onto the plane in time even though their luggage has already been loaded. This essentially means that somebody has to look through all the (already loaded) luggage to remove that specific piece, which, depending on how deeply it's buried, might take considerable amounts of time. This isn't a problem with hold luggage, since there cannot be any hold luggage aboard a plane without a corresponding person.