A common misconception, soccer is actually incredibly popular in the US. The trick is that while traditionally, popularity is measured by television numbers, in a digital and far more diverse era, soccer has thrived in mediums not traditionally measured.
But let's start with television anyways. This past summer, the FIFA World Cup Final drew over 5 million views on just the Spanish broadcast of the game in the United States. That's more than any game of the 2018 Stanley Cup Finals or World Series. And that's always been a huge part of inaccurately measuring soccer success; English television ratings, the traditional measure, don't measure all of the fandom. To truly gauge soccer's popularity, you have to combine multiple language broadcasts for the whole picture. Digital streaming is also not included in most traditional television numbers. This past FIFA World Cup saw US streaming numbers at an all-time high, including over 5 million for just Argentina's opening match. So people are watching soccer more than a lot of sports in the US, they're just watching in ways we're not used to measuring.
Another key in the misidentification in soccer fandom is the plethora of soccer content and how to accurately measure a fan who just watches one league or a fan who watches all leagues. In the US, every major television network has a TV and streaming deal with a different soccer league. If you aggregated all these viewers and streamers into one number, soccer immediately becomes the most watched sport each week. However, that feels disingenuine and we haven't seen a perfectly accurate number yet. What we're stuck with is each network and league proudly promoting their US viewership, but the numbers make soccer look smaller than it really is.
Finally, the FIFA video games series made by Electronic Arts (EA) for gaming devices has drastically improved soccer's reach in the US. The FIFA 18 version of the game for the 2017-18 soccer season sold more than EA's National Football League game at over 2.1 million copies in the United States. Many believe that the prominence of this game in the under 25 crowd is redefining soccer fandom, as kids are able to play as teams all over the world and build custom teams, allowing them to follow players rather than teams.