But it’s good to see Messi enjoying football again, even as an English fan as I’d rather him have come to the Premier League
Well with football fun is related to success.
If you're stressed you don't play well, if you don't play well you lose more games and you get even more stressed.
It's a team sport, far more teamy than most uniquely British or American games. The press, the fans, the owners obsess over celebrities but mangers, coaches, and players know that two weak links break the chain and a tactically creative pass is worth more than the difference between the best striker and the 10th best.
Wherever Messi goes people expect him to obliterate the opposition and win championships. In other words he's on teams where everyone is on edge, every fan ready to turn on him (and the other stars), every coach one loss away from being sacked (see I know UK English too).
Every time he went to play for Argentina was the same dynamic magnified. Throw together a bunch of stars, they have no time to build chemistry as a team and then people can't understand why their linear addition predictions don't come to fruition.
Again, I think the players know this better than anyone and the ones who love them game value being on a team with good chemistry and attitude over just about anything else. They may take offers and stay places for money, but their heart is in the game not the bank.
When Messi shows up on the worst team in the USA there is no pressure, no drama (You've got to believe every original member are worshiping him), and let's be honest a lower level of skill. There is no where to go but up and no expectation but improvement. It won't last forever, nothing does; but he'll get to feel the love of the game until its time for him to retire with his millions and think about what he'll do with the rest of his life (probably coach).
Another obvious factor that I haven't seen remarked upon is that Miami is a city where you can easily get by with just Spanish. Obviously Barcelona is similar but I'd be willing to bet Paris, Arabia, or UK aren't like that and maybe in Paris it annoyed Messi & kin. (Hec the French wouldn't used Spanish even if they knew it)