Because of the issues everyone is having with the low inventory weight limits (when compared with what you might want to carry for materials & upgradable armors, weapons, etc combined with what you might pick up on one trip out of town). It's a good way to mitigate the issue without doing away with weight limits entirely.
Storage boxes are not a good way to solve the issue, because you still have to travel back and forth to them to use them (loading screens, etc). Of course they're nice to have too for the pack rats out there.
