@gap, probably they haven't reached the point where the only use for the resources is to upgrade weapons in tier mode.
as for OP's suggestion... i doubt that'll happen. cuz there is buyable resources pack for credits. so if you could transfer them easily - less people would buy the pack and so less money to ubi.
For myself, I had completed all the upgrades long before I finished Fallen Ghosts and there's no other use for the resources on that mode - meaning they're all just sitting there useless. To me it would make sense to have the option in Fallen Ghosts to be able to move them over to a completed main game save simply so that they can get used.