Additionally, while Churchill is a good location for an "exit" port for Prairie grain, it is out of the way where patrolling the N.W. Passage is concerned.
Churchill is a little over 600 NM (1,000 km) from the entrance to the Fury and Hecta straight. But that doesn't get you into the passage. It only gets you into the Gulf of Boothia and you still have another 400 NM to go to the actual passage. All this is through the Foxe basin/Fury and Hecta straight/Gulf of Boothia area, which is one of the most ice congested area of the Arctic and one of the least likely to be ice free. You would be fighting all the way.
If you want to use Churchill and avoid that, then you have to exit Hudson Bay altogether, transit out of the Hudson Straight to go between the Northern tip of Labrador and the Southern end of Baffin Island, then North along the outside of said Baffin Island. Problem is, if you do that, you may as well go to St. John's, which would then be only about one hundred and fifty NM further than Churchill but will always provide you with full support where maritime services are concerned.
I wish more people would use a globe when looking at Arctic matters: It provides a much more realistic overview of the real navigation challenges.