I've long thought the whole "MLS will eventually rival or eclipse the NHL in popularity" idea was a complete sham.
One, that league itself is a sixth-tier mess. The NHL is the best hockey league in the world, the MLB is the best baseball league in the world, the NBA is the best basketball league in the world, the NFL is the only American football league in the world. The MLS is at the fringe of the world's 10 best soccer leagues. It's not a great product, and its league/pay structure is not set up in a way that will allow it to rise up higher.
Two, sports are embedded in culture, and soccer as a mass spectator sport has just never been a considerable part of American culture. Yes, tons of children play it, but they have forever. That hasn't translated. It's a deliberate and nuanced game that isn't easily packaged up and commercialized: It isn't cut into bits for our TV viewing habits, it produces few "highlights" in the sense we imagine them. I liken it to like, K-pop and Bollywood films. Of course they're enormous global phenomenons with devoted fans in the United States, but they're not going to replace mainstream American entertainment on a grand scale because they are of the cultures where they were born. This is all a part of, actually, why hockey is a distant 4th in this country, too, but there's at least roots in a lot of northern states and tradition of success in converting viewers throughout the country.
We did do soccer 150 years ago, and then we started picking the ball up, and then we started throwing it, and then we just bastardized it further and further until we got the sport we liked/deserved in American football.