Soccer is extremely widespread and played by both boys and girls at a young age (up until puberty or so), but there definitely is a gender gap after that. I'd guess that there are a lot of other sports that are almost solely played by boys, so boys tend to drift away from soccer, while there are fewer options for girls. (Though there are some - lacrosse and softball for a couple examples.)
Field hockey is almost exclusively a girls sport in the US, while boys have (American) football in the fall. Both draw from the potential pool of soccer players in US middle and high schools.
