I think the problem is we've almost tried to be too serious and as Bonzo said, it's not a national sport. It is taking hold and fast it seems like especially with all the new expansion teams coming to the MLS (I didn't follow the MLS until we picked up Atlanta United). It is nice to see "soccer" specific stadiums being build for teams in the MLS. I think with the UK, someone correct me if I'm wrong, there's not a plethora of sports people choose to be fans of, just multiple teams from the same city. Look at LA for example. They have five different sports leagues in town, MLS, MLB, NBA, NFL and NHL. Each of those leagues have two teams each. That's not to mention that San Francisco and San Diego aren't but a half day drive away and they also have a hand full of teams. In Atlanta we're lucky enough to only have one in each and our NHL team left a few years back due to the lack of a fan base.
With that being said, the average attendance to soccer matches has exceeded NHL, MLB and NBA in recent years. Atlanta set a record for season ticket holders before we even kicked a ball. I believe Austin has broken that record with their new expansion team. We, Atlanta, also have an average attendance of 53,000 per game which is crazy and from everything I've seen on the interweb that is 19th worldwide for football attendance.
I think it'll be a while until we're truly serous about soccer and are considered a formidable squad to challenge anything. Two World Cups ago we were lucky to make it to face Belgium in the round of 16.