I am on guessing here, but guessing based on having graduated high school in Canada, attended university in the US, and having relatives attend university in both countries.
I would guess that the most important issues are the high quality of the universities in Canada, the relative better affordability of university in Canada, and the fact that admissions to top universities in Canada tends to be predictable and based largely on merit. In contrast admissions to highly ranked universities in the US is relatively much less predictable.
Another issue might be that at least in my experience companies in Canada seem to prefer to hire graduates from the very good universities in Canada. Having graduated from highly ranked universities in the US (MIT and Stanford) I at least found it difficult to find employment or even job interviews in Canada.
If you can get accepted to Toronto, McGill, or UBC, and if you have Canadian citizenship or permanent residence, I do not see much point in attending an average university in the US. There might be some point in attending somewhere like Harvard, Princeton, Yale, MIT, or Stanford, but admissions to these schools is very difficult for international students and is also very difficult to predict.
And students choose which university to attend for a very, very wide range of reasons.