Lake Ontario features a very dense area from Toronto to Hamilton, but on the US side there’s really just Rochester - hardly major at this point and came into existence due to the Erie Canal as much as anything else.
The other lakes that the US and Canada share feature nothing prominent from Canada, but Buffalo, Cleveland, Toledo, and Detroit in the US. Why aren’t there other Canadian ports along the lakes? And why doesn’t northern NY feature more?
I guess I should clarify that I’m not asking the hard-to-answer “why’s didnt X happen” but more “why did this happen the way that it did.”
The biggest reason why is the arable land in southern Ontario. The vast majority of Ontario's population is in the area that is shaded in red (arable land). This area is bordered by Lake Ontario, Lake Eerie, and Lake Huron. North of this area is the Canadian shield country, of which, very little is arable land.
The Canadian Shield is unlike most geographic formations on the American side of the Great Lakes. In fact, the only states who have this geographic formation are Minnesota, Wisconsin, small parts of the UP in Michigan, and a very thinly populated region of Upstate NY (the area you're referring to which has few people). This geography is composed of rocky soils inadequate for growing most crops, rugged rocky outcrops and hills, and literally millions of lakes. It is very expensive and difficult to build infrastructure in this landscape, and this landscape is very rugged and inhospitable (albeit, very beautiful to look at). So this is why there are no major Canadian metropolitan areas in northern Ontario. You'll notice the American side of the Great Lakes has an enormous amount of arable land, multiple times the size of southern Ontario. IN particular, Ohio, Michigan and Illinois portions of the lakes. Unsurprisingly, this is where most major American metropolitan areas were established.
So, why Toronto? Part of the reason is luck. In May 1793, Lieutenant-Colonel Lord Simcoe, the first lieutenant-governor of the newly organized province of Upper Canada, visited the area around what would be Toronto for the first time. Simcoe was unhappy with the then-capital of Upper Canada at Newark (near Niagara Falls), and proposed moving it to the site of present-day London, Ontario. He was dissuaded by the difficulty of building a road to the location. Therefore, in 1793, the Toronto purchase site was chosen by Simcoe where the Humber River empties into Lake Ontario. From the early 19th century onwards, Toronto would be the administrative capital of Upper Canada (Ontario). It could very well have been London, Ontario, or Windsor, or Sarnia, or even the original site near Niagara Falls.