Yes, that. The differences between US and Canadian football are not just cosmetic. In addition to the differences mentioned, there is that little fact that Canadian football is played on a substantially larger field. You can't just expect a CFL team to come to the US and suddently adopt US rules.
Further, in response to one poster's comment about a "play-in" opportunity for the CFL champ to enter the NFL playoffs, why should a Canadian team even want to consider it if they had to play by US rules? Also, why do people automatically presume that, if the CFL and NFL "merged" they would have to adopt US rules? Here's an odd-ball idea, have CFL-NFL games use a home-rule system. When in Canada, use Canadian rules, when in the US use American rules. Believe or not, Americans are smart enough to learn two sets of football rules. After all most CFL players played US collegiate rules before joining the CFL.