It isn't. Polls don't tell the whole story. If I was polled about liking tennis, I would say yes, since I appreciate the sport, but haven't watched a full match since Fed/Nadal some 10 years ago. I'm sure a bunch of soccer moms respond, "yes, I'm a soccer fan," because they're fans of watching their little Jimmy run around aimlessly on the weekend, but likely never watch an actual soccer game on television. MLS ratings are abysmal, getting outdrawn by the in' WNBA. EPL ratings don't fare much better. No professional soccer match has even come close to doing what the Stanley Cup Finals draw. World Cup ratings are always solid, but people watch for the event, not necessarily because they're avid soccer fans. Just like the Olympics.
When the MLS Cup can draw 5-7 million, then I'll say soccer has arrived as a "Big 5" sport here. If you factor in individual sports, Soccer is 7th or 8th here on the pecking order.