Honestly, I think a big (but not sole) reason soccer is so unwatched to Americans is that that the game never got popular at the college level.
Ignoring MLB (which grew on its own even if the college game I'd midly popular in its own respect), one of the big reasons the NBA and NFL are even popular is because the college game popularized the sport so much. I think some people like talk about point ti the atmosphere and rivalries of pro European and Latin American soccer and that American pro sports are mild in comparison, but they forget that those same viscous rivalries exist at the college level, which in the case of college football is way more popular than the NFL, instead.
Then look at the smallest of the Big Four of American sports: the NHL. Hockey's collegiate game is basically non-existant popularity wise. Again, I'm not saying that's the sole reason, but collegiate sports are incredibly integral to the popularity of sports in the US. Collegiate soccer is really, really bad, so until that improves, the MLS is never gonna have a dearth of talent or eyeballs, imo.