Was watching a few old highlights of the Brief from the 50's today, and found that Northern Ontario always had their own entry, even then. Is there a reason why even thought it's not it's own province, it would be given an entry as if it were in events such as the Brier and the Scotties?
It's history more than anything.
At the first Brier in 1927, six provinces were represented – New Brunswick, Northern Ontario, Nova Scotia, Ontario, Quebec and Saskatchewan, along with teams from Toronto and Montreal. In 1928, Alberta and Manitoba joined the championship. In 1932, city representation was dropped, but in 1936, Prince Edward Island and British Columbia were added, followed by Newfoundland in 1951 and Yukon/Northwest Territories in 1975. Source
Northern Ontario has always felt it is "different" from Southern Ontario in a lot of ways, geographically as well as socially and politically. There have also been movements to try to get itself separated from the rest of Ontario, but they have failed. Overall, the people of Northern Ontario believe they have a vastly different identity than Soutnern Ontario, and they want to keep it that way. Source
Keeping their separation in events like curling (and specifically the Brier) help keep that identity alive.