Exclusive control over the supply, and lower costs — same as business operates now.
It makes no sense to make a dangerous trip across the sea wasting time and money when the same product is available right on your doorstep.
Tin was rare, copper was common. For tin you would make the voyage. In fact they did, to the British Isles and to the Black Sea area. But they did not travel for copper. There was copper in the Sinai. There was copper in Asia Minor. There was copper in Europe. There was copper in Africa. There was copper in the Middle East.
I'm open to Phoenicians visiting North America, but it would be quite an undertaking to have them work these mines. Plus, there are rapids and cataracts on the St. Lawrence where portages would have to be maintained. It would be an enormous effort.
An easier theory is that the Native Americans worked the mines because they heard of markets on the East Coast and Central and South America.