The problem is the MLS will never be a top-tiered professional soccer league. Just like European basketball leagues, the best players are going to want to play in the NBA, just like the best soccer players are going to want to play in Europe.
Even within Europe many domestic leagues suffer, because their countries’ best players want to play in England, Spain or Germany.
The best players play in England, Germany and Spain because teams in those leagues can pay the most to the players.
Also, quite a few players for the U.S. have duel citizenships, especially as sons of former American servicemen married to German women.