Football (soccer) is globally popular in the modern day because of British Imperialism during the 16th and 17th century. It remains popular because it's incredibly fun to play and watch. If you want to understand it more, go play soccer. It's fun, and it's good for you. Cheers.
Soccer is played by heaps of kids but the main soccer league (A-league) is pretty much a joke. The main sport in (most states) is Australian Rules Football. The grass-roots (AusKick) get decent enough participation but The main league (Australian Football League) gets ridiculous attendances and viewership both in absolute, and even more so in ...
The real reason soccer is so popular is because it's one of the cheapest and simplest sports that anyone can play. Just need something that resembles a pitch (small as it may be) and a ball, and off you go. I once played soccer with a ball made from a bunch of of plastic bags wound around a block of wood.
One reason soccer is as popular as it is for young kids is because it sort of 'works' even when they don't know how to play. The kind of amoeba-ball they naturally tend toward still allows them to get exercise and some very basic skill building with ballhandling and body-control.
American football is relatively popular, not like soccer, in Canada, Mexico, and England. Less than an hour and people are already defensive about nonthreatening questions from the US. The US market prefers commercials every ten minutes, hard to do in soccer. I came here to say this.
Something you should add is that soccer is really popular growing up here. We have a ton of rec leagues, indoor soccer, and it's quite popular. A lot of kids grow up playing soccer in the Northwest, so it's only fitting that the Sounders get a cult following. It's really not as big as it's going to get yet, so we'll see where we are in 10 years.
Structurally, the closest popular American sport to soccer is probably basketball (because play is mostly continuous), but even then we take the concept, shrink the play area so as to force more contact, and redesign the scoring method to allow for 1) scoring more often, and 2) higher rate of shots which are visually impressive.
Thanks to sponsorship and marketing that makes soccer so popular and brings 4 billion people to watch it. Soccer is a sport in which so many companies invest. You will get to watch it everywhere you go whether it is a restaurant, airline, bar, or a videogame. However, soccer clubs Barcelona and Real Madrid are the biggest sponsors of soccer.