In a recent American survey only 47% of the Super Bowl viewer actually watched the game(Commercials, HT show doesn't count) and 53% don't care about the Super Bowl game(Ads, HT show, and Togetherness). If you do the math only 50.76 Million people watch the game compared to 29.2 Million Americans that watch the 2014 FIFA World Cup Final. It's getting there. When the MLS becomes a great league, it will be really popular. What I know is soccer will be popular in the US as it is popular (already is) in other countries.
Soccer will become as popular as it is in other countries because the populations moving to the U.S. are often from 3rd-world countries where soccer is popular. There will be a forthcoming spike in its popularity, and as we have all seen in the past ten or so years, the World Cup has already gotten more coverage.
I think the popularity of soccer is steadily growing in the United States and I think it will continue to do so in the future. I think if we developed a national franchise for soccer we would probably see that interest increase dramatically. After all our other sports started out this way as well.
Soccer will never be as popular in America as it is in other countries. The game format doesn't leave enough room for televised sponsorship, so it would be underfunded for years before being a mainstream spectacle. In addition, soccer is a game of finesse, rather than big moves. This makes it a thinking man's game, similar to amateur wrestling. In general, American sport fans want simplicity, and giving them a complex sport to watch makes it harder for fans to really dig in.
I think that a lot of sports are popular because of the history of the culture. Soccer is one of those. America grew and was raised on the idea that football (american football) was the way to go. We never cared much for soccer and in most cases, people view it as a rather weak sport. So, soccer will never be quite as popular to the U.S. as a whole, but there sure are some soccer fans running around!
I think that other sports overshadow soccer in America. In many regions across the United States, there are several professional teams. The 4 major ones are hockey, basketball, football, and baseball. These sports are already very well established. There are stadiums, leagues and teams. In order for soccer to become as popular in other countries, the U.S. would have to establish a similar following as there is in the 4 major league sports. This would require building new stadiums or sharing existing ones. If we use existing stadiums, conflicts may occur with other sports. Furthermore, because the 4 major league sports have somewhat alternating schedules, it would be difficult to get new fans to turn away from a currently popular sport in session.