Again, I think you are defining 'tourist' a little too narrowly... and as has already been said there are about 15 to 20 million people annually who are visiting the Niagara Region, either via Toronto or directly from the USA so the idea that Toronto is the lone tourist magnet in the area is completely erroneous
The discussion here makes it seem like only tourists will visit the aquarium which is nuts. Odds are more people will visit if it is located in heavily populated area that's easily accessible by many means. That puts the CN Tower site ahead of Burlington, Niagara and virtually anywhere else in Ontario.