If you think about it, the polar regions don't support much in the way of agriculture. Sailors in the temperate oceans could hope for a variety of other things to eat-plants, fruits, land animals... In the arctic north, however, food mostly comes from hunting and fishing. The Stellers sea cow was simply easy prey in a place without many other choices. It had also been hunted long before fur-traders and other Europeans came into the northern Pacific, and the population remaining in 1741 was limited to a few islands inaccessible to small craft. The European sailors simply finished what had begun long before.

There is also a theory that the population was reduced by the early hunting of sea otters. It's long been demonstrated that sea otters keep the sea urchin population in check, which in turn allows kelp beds to flourish. Without sea otter predation, sea urchins would have damaged kelp beds, which were the feeding grounds for the Stellers sea cows.

The range of the Steller's Sea Cow was limited to the Commander Is. in the Bering Sea. The Manatee and Dugong range over very large areas of tropical coasts and rivers - the Caribbean, West Africa, the perimeter of the Indian Ocean, Red Sea, Indonesia, New Guinea - they are threatened, but their native range is not limited, in the way the Steller's Sea Cow's was.