Some current gear is made untradeable, unsellable, and undesynth-able in order to discourage people from rolling on gear they're not gonna equip.
After doing Emerald EX I'd roll on every single weapon if I could liquidate them into gil, seals, mats, or whatever. But instead they've set it up like this so those items generally flow to the people that will use them.
If you want to hold onto them, as they become less current they'll usually become sellable.