Historically, the United States were always very nationalist, they resist conforming to what others do unless completely necessary, even if the way the rest of the world does things is demonstrably superior.
It is understandable that they would keep to their national sports instead of the world-wide ones.
However, it's changing now. The Latino population in the USA rises, soccer is the 2nd most popular sport for people under 24 and the MLS is getting better and better. i believe that in 20 years, soccer will be the 3rd most popular American sport behind baseball and gridiron.
Not a chance that it will pass basketball anytime soon. It will have a difficult time even passing the popularity of sports like hockey, tennis, and golf. The best case scenario is that it might be the 4th largest sport in 20 years.
I didn't read all the pages in this thread, so maybe someone already mentioned it, but: If you talk about the US and not the US + Canada soccer may already have reached or passed hockey according to various metrics:
1. Studies asking people about their favorite sports. Afair about 4-6% say that hockey is their favorite sport, while about 3-6% say it's (men's) soccer. The numbers for the latter depend on whether women's and men's soccer are grouped together and whether it's a World Cup year or not. The number of people saying they like/watch the sport is also very similar (I think about ~60% for both, but I really don't remember). But there's one similar metric where soccer is still far behind. About ~10% say that they are avid fans of soccer - meaning that they really follow the sport but it may still not be their favorite sport - and I think that number is about twice as high for hockey.
2. TV revenues: According to Wikipedia the TV revenue for the NHL in the US is around 200 million dollars per year (in the US+Canada it's 600 million dollars). For soccer we have to add the revenues of various leagues and events to get an accurate number, while that's not really important for hockey: The English Premier League TV revenues in the US are estimated to be ~100 million dollar per year. The MLS rights are ~90 million per year (the package also includes the rights to air some USMNT games). The TV rights for the next 2 men's World Cups are estimated to have sold for ~425 million dollars, making it ~50 million per year. I don't know how much revenue the women's World Cup and Mexican Liga MX TV rights add to this, but it's probably somewhere in the lower 8 digit numbers. So in total we are looking at national TV revenues of ~200 million dollars for hockey and about ~250 million dollars for soccer.
3. Attendance: The average attendance at MLS games this season currently stands at ~21.500, while the number is at only ~17.500 for the NHL. But of course NHL stadia are way more limited by their size. And due to the larger number of teams and games per team the total attendance in the NHL is way higher than the one of the MLS: About 22 million vs about 7.5 million. The difference gets smaller when you exclude the Canadian teams in both leagues, but is still very noticeable. Also a lot of US based soccer fans actually follow the English Premier League or the Mexican Liga MX and do not actually attend any games in person. The TV ratings for the EPL are about twice as high as the ones for MLS, and the Liga MX is also slighlty ahead afaik. This last point brings me to:
4. TV ratings: It's getting late here and I have to go to bed so I won't double check the numbers. But afaik the average NHL game is watched by ~350.000 on national TV. EPL draws ~400.000, MLS about ~200.000 and Liga MX somewhere in between. The NHL playoff ratings are way higher but so are the World Cup ratings. Both should cancel out somewhat.
Note: Sorry for me bad English. As I mentioned it's getting late and while I have no problem reading English writing it is way harder for me. That's why I usually don't post much in English language forums. Oh, and I have no idea how popular tennis and golf are. The same goes for NASCAR, which should also be popular in the US. It's just difficult to compare team sports to other sports.