Toronto has some major hurdles to overcome, not the least of which is the fact that half the stadium is empty for the Bills games that have been held in Toronto and it's unclear whether the city truly cares about the NFL.
There's no NFL-ready stadium in Toronto, so a new one would have to be built. The Rogers Centre only seats ~55,000 and it couldn't be used for more than a year or two as a temporary home. There's simply no money for a new stadium. There is no chance in hell of public money being used to build an NFL stadium (Canadians don't really go for that), so unless somebody magically shows up with a billion dollars to spend, it ain't happening. That person does not currently exist. The corporation that initially seemed to want to buy the Bills and relocate them, Rogers, has switched gears and is now pursuing the Raptors and Maple Leads (the true cash cow franchise in Ontario). The NFL no longer seems to be in Rogers' plans.
There's also the problem of TV rights. A franchise in Canada would have to negotiate television contracts independently from the rest of the league. The primary sports broadcaster in Canada currently has a massive stake in the CFL and would likely do everything possible to keep it alive, since the CFL performs very well (better ratings than the NFL) in most of the country and is far more lucrative than the NFL would be (last I checked, American broadcasters spend so much on NFL rights that they lose money).