Even though there are a lot of Bills fans in the GTA, I still believe that series failed because it was the Buffalo Bills. I am of the firm belief that Toronto would get behind a Toronto NFL team, it's a great sports town as long as you're either a. a winner or b. playing hockey. The CFL is such a non-starter in modern Toronto that it's not a great barometer for the potential success of the NFL (you didn't say that, but that's the argument some use).
This is all conjecture and hypothesis, and not to sound like I would support the idea of the NFL to Toronto, but the way some Torontonians think about the city of Buffalo... to me it is no surprise why those "home games" failed. Toronto does not care for sloppy seconds, and frankly it's the same reason why I think the Argos don't do well in Toronto anymore... it's not the big event.