Bear with me, this is a long post. For the sake of this argument I call Soccer: Football and Football: American Football.

The spread of football was immensely influenced by the Brits. The fact that there are ex-colonies which have other games that are more popular is not contrary to this. This is because of economic and social reasons.

Case in point. To assume football is not immensely popular in India is very wrong. Football does not get the exposure and funding that cricket does, hence its pathetic state of affairs in India. There are football clubs in India that are older than many of the biggest clubs in Europe, Mohun Bagan AC, 1889. There are local derbies that regularly draw crowds of a 100,000. In the 1940s and 50s the Indian football team regularly won medals at the Asian level. They were amongst the top few teams in Asia. The demise of football in India happened in more recent years. A young developing nation, where playing sports was secondary to obtaining a means to livelihood, won the Cricket World Cup in 1983, in England, beating the then unbeatable West Indies. It was classic David versus Goliath. The whole country went bat sh** nuts and the government made cricketers into heroes. In the following decades when the economy expanded cricket was already in the drivers seat and marginalized the economic access of all other sports. Cricket grew in India at the expense of every other sport. Football and Hockey suffered the most. Sports in India have never recovered since then.

Similarly, I would argue that all the different rules of football came to America with the Brits. The then Americans took the rules and modified them to their liking, coming up with a completely new set of rules which became popular in America when the US was vehemently American and had a very "us versus them" attitude towards the rest of the world, especially Britain. America was largely isolated from the rest of the world, politically and socially until WWI and WWII. If not for the Japanese bombing Pearl Harbour it can be argued that the US could have stayed neutral through the war (but that's a different tangent). My point being when the economic expansion happened in the US American Football and Baseball were already in the drivers seat and marginalized the economic access of all other sports. Thankfully in America the economy was big enough that there was some moolah to go around for the other sports and they did not get completely destroyed.

I am not saying the above are the only reasons that influenced the respective cases. There are other factors that influenced them too. But that would be a larger discussion.