Aside from ownership groups to get teams up and running, you can't leave out media rights. Also, scheduling in terms of being during the NFL's season or during its off season. All of this would have factored into the eventual success or failure of the league.

I also think that somehow we somehow can't really have two competing pro leagues in the same sport at the same time as one will get absorbed by the other in the end game.

As far as that CFL in the US thing. I don't recall exactly why the did that rapid US expansion and almost as rapid contraction of all those franchises. Personally, I thought it was a good move by them in terms of getting some popularity outside of Canada and since they played opposite the NFL schedule they wouldn't have any eyeball or attendance issues in terms of folks saying to themselves do I do to the NFL or the CFL game today. And from my memory, they went into NFL cities or cities that had NFL, USFL, or WLAF cities so its not like they didn't go to cities who didn't support pro football.