It's both surprising and not surprising. Surprising in the sense that sports are VERY popular in America, especially among adults. Logic would say that if sports are popular, one could make a show around it to get kids hooked early on in life, thus getting more sports fans (and further thus, more money for the sports franchise in question).
It's not surprising in the sense that sports shows, by their very nature, tend to be serialized. Yes, every episode could focus on a different match, but in a larger sense, the team is working towards a goal. There's no reset button at the end of every outing. Serialized animated TV shows are a rarity in the U.S. And genres outside action and comedy are even rarer. Not saying it's fair; I'd like to see some more diversity too, but that's just the way it is right now.
Adding onto that, it's not like the sports genre doesn't exist in the U.S.. It just happens in live action. Whether this is a bias against animation or a fear of oversaturation of the genre or just simply sticking to traditional ways of doing things (or all three), that's unclear. But that's just how it is. Americans have watched live action sports films/movies for years and it will take a real breakthrough show to get people interested in the animated side.