The problem is that building highways in Northern Ontario is a costly endevour.
From the MB border to Sudbury(the closest 400 series highway connection), highway 17 is over 1500km long. It's entire length is over 1900km. Of that, only around 100km is divided.
Now, in Southern Ontario, or on the Prairies or most of the USA you do not have the hard rock of granite to blast through.
We are talking in the neighbourhood of $10 Billion to do it.
http://www.thepixelwarehouse.com/69_...tionPlan_e.pdf
It would be hard to convince the various levels of government to spend that much money up in Northern Ontario.