Yep. Sport isn’t popular enough here to sustain that anytime soon. No one cares about the lesser leagues and teams would lose support if they got dropped down. I can see them adding it way down the road if the sport grows a lot in popularity here.
Other major difference is it has a playoff after the year for the MLS Cup and that’s viewed as the true champion and not the team with the most points in the regular season (Supporters Shield).
That reminds me about my thoughts on this concept. As much as I love playoffs in American leagues/sports, and as exciting as they can be, I don't think they are a good reflection of the best team - the team who ought to be considered champions of a given league.
The long haul ought to matter more in sports in the US. A team gets hot at playoff time and suddenly they are champs?!? Granted there is an appeal to that as well, but I really prefer the team who endured and thrived over the long-stretch being recognized as champs.
Although it would be tough to establish such a champion in American football simply due to the limited number of games. There's no way every team can play every other team even
once in a season, so I suppose there is no other solution in pigskin. But in all the other big sports, teams play so many games, it would be easy to recognize the champion as being the team that ended up with the best performance over the long haul.
Edit: And I always loved promotion/relegation. The average US sports fan would never go for it though, for sure.