[Capsicum]

What factors could have led to the other lake shore (of Huron and Superior) regions of the province of Ontario be more populated, so that at the very least they would have six-digit rather than 5-digit population numbers?

Let's say have places like Owen Sound, or even Sault Ste. Marie reach the six digits. Thunder Bay already does, but most other Lake Huron and Superior cities don't approach its population much. As mentioned by Docere, stateside, Green Bay, Wisconsin, on Lake Michigan is a pretty big city (Thunder Bay sized) and is roughly about as far north as Owen Sound.