Soccer will always struggle in the US because of culture. Outside of the US, soccer is the sport everyone gathers around the TV to watch, but here in the US that's usually the NFL. If American Football never came to existence and we played soccer like the rest of the world I bet that the US would have dominated and we would have clubs like PSG and Real Madrid. But because we have American Football here, all the money and attention is on that sport and soccer is just a niche sport.

The MLS can do alot to become more popular but it won't ever be what soccer in the rest of the world is because to do that they would have to dethrone American Football which is just never happening.