Good point
You still haven't told us what your theory is.
It's already been said in the thread by other members: you love what you grow up with. I think, that, you have a sport like soccer that has been around for a long time and you mix that with the way Europe and the rest of the world (minus the USA) was kind of held together, it's inevitable that soccer is as popular as it is globally. Europe controlled most of the world for a long period of time. It's just natural that thing would spread. Look at the Meiji period in Japan. Now, I know the Japanese do everything to the ^n, but they went through radical changes based on their trading with Europe and the Americas.
So, what happens is what you have generations of clubs being passed down, fan bases along with them and you get the climate we have today. It's 100% the same in the USA. Different sports of different ages, but the most popular (football and baseball) have very old roots in American society. I mean, the US would ask suspected German spies who won the World Series the previous year (to see if they were truly American or not).
EDIT: Look at how nations like Cuba, the Dominican Republic, Panama and Venezuela churn out top-tier baseball talent year after year. Most of those countries were under US rule at some point.
I could probably write a dissertation on the subject, but I haven't the time. I hope the point is understood, however.