NPS website

The national park website offers a wealth of information on trip planning.

http://www.nps.gov/isro/index.htm

Yellowstone vs. Isle Royale

Per the National Park Service, the yearly number of visitors to Isle Royale is less than Yellowstone receives in a DAY!

However, the Island's per acre backcountry use is the highest of all National Parks in the United States.

Why is Isle Royale not highly visited?

Isle Royale is the least-visited national park in the lower 48. Only two remote parks in Alaska rank lower. 

Why is this? Okay, here are my thoughts.

1. It is very difficult to get to. You must take a ferry or seaplane. You cannot drive or even hike there.Now while the same is true of Dry Tortugas National Park (located 70 miles from Key West on a small island), Isle Royale has some additional factors.
2.  It is not near any other attractions or population centers. Dry Tortugas, mentioned above, is a short flight from Key West, which is a tourist mecca. The jumping-off points for Isle Royale are Copper Harbor, Michigan; Houghton, Michigan; and Grand Portage, Minnesota. I'll venture to say that no one has even HEARD of these places, let alone taken a vacation there. 
3. It is not on the way to anywhere. Houghton and Copper Harbor are on the northern tip of the Upper Peninsula of Michigan (600 miles from Detroit). Grand Portage is a few steps away from the Canadian border (and 300 miles from Minneapolis).
4. It has no golf course, gourmet restaurant, shopping center, or television.Yikes!
5. You can't really go for just a casual visit. While it is possible to make a day trip there, you would be spending 6-7 hours on a ferry, and only 2-3 hours on the island. So most visitors spend multiple days there.
6. Although a fabulous place, there are a limited set of activities available, and these may not appeal to everyone. These include: Camping, hiking, fishing, canoeing, kayaking, wildlife viewing, and boat tours. (And perhaps a little, um, photography!)
7. You will not be catered to, and must expend your own energy. You may think you are on Gilligan's Island with "not a single luxury." Per the National Park Service, "This Wilderness Island contains a road-less backcountry which prohibits the use of all wheeled vehicles and devices (except wheelchairs)." So no, there will not be a limo waiting at the dock to pick you up. 
8. You probably don't know anyone who's been there before. Your uncle or your neighbor may have been to the Grand Canyon or even Paris and Bejing, and come back with pictures and stories. Whereas very few people know Isle Royale even exists. 

The Next Frontier

So yes, Alaska is the last frontier. But due to popular cruise ships, Alaska national parks such as Glacier Bay receive far more than the one I want to visit next.

Glacier Bay visits in 2012: 454,337
Isle Royale visits in 2012:    16,663