tuna is more abundant than salmon (so cheaper to tin at source) but doesn't last as long out of the sea (so more expensive to ship). I am only expressing an opinion on the 2nd statement as i don't know it as fact, but i think i am right in saying that uncooked tuna doesn't last as long as uncooked salmon.
ps. location has a lot to do with it - tinned tuna tends to come from poorer countries where they pay 10 cents an hour and no toilet breaks, adding the fact that they have to ship it a long way if fresh, whereas salmon in the UK generally comes from Scotland which is more expensive to tin (as we don't allow slave labour - in theory) but obviously a lot cheaper to ship fresh
In the UK fresh salmon is much cheaper because of extensive farming around the UK whereas tuna has to be imported. As stated in a previous answer, tinned tuna comes from "cheaper" countries but tinned salmon tends to be the wild stuff from Canada etc.