Diamond League in Stockholm had a ton of empty seats as well. The sport isn't very healthy right now. Too many drug busts, too many years of East African dominance, and too few compelling stars that people are clamoring to see.
An international friendly in soccer can fill a 60,000 seat stadium.
The NFL has a packed house to watch two teams with losing records play.
Even baseball draws better for a slow moving sport with sporadic action.
After 45 years of being an athlete, coach, and fan of this sport, I find myself bored with it. The product and the presentation is rather meh. I'd rather attend an MLS match because there is chanting, singing, drumming, and actual passion on display. That makes it fun and a party atmosphere! I am skipping NON because the stadium will only be 1/3 full and the only noise will be the occasional cheering, some Nike commercials played too loudly, and some over eager field interviewers trying to be hip.