The NFL season is over, but the trouble might just be getting started for the league.

The NFL saw a sustained drop in viewer interest this year that isn't easily explained by the US presidential election and the frenzy that surrounded it.

Before the election, ratings for NFL games had declined 12% year-over-year, according to analysts at MoffettNathanson. And for much of the season, every NFL primetime offering — on Sundays, Mondays, and Thursdays — was down by double digits.

People floated a lot of theories about why, from cord-cutting to injuries to Colin Kaepernick. But the party line at the NFL was that the decline had to do with "unprecedented interest in the presidential election."

The fact that NFL ratings improved slightly after the end of the election seemed to support that theory.

"It's an encouraging rebound," NFL Commissioner Roger Goodell told ESPN in December. "I think it proves that the election was certainly a factor."

But the election doesn't explain everything.

Not just that

After the election, including the playoffs, ratings were still down 5%.

For the whole regular season, they were down 9% in total, and down 6% for the playoffs, according to MoffettNathanson. Ratings were also down versus the 2014 season, which Recode's Peter Kafka notes might be a better comparison, as 2015 saw a huge NFL ad war between the daily fantasy startups Draft Kings and FanDuel.

The new TV

What the NFL likely doesn't want to consider is the possibility that the entire concept of "must-see" live events — those that will always draw massive TV audiences — could be under pressure.

In 2016, NBC's Olympics primetime broadcast was down 17% versus 2012. The damage was even greater among younger people. Among 18- to 49-year-olds, there was a 25% drop-off in audience size for the bulk of the games, according to Bloomberg Intelligence.

While some awards shows have stayed steady, others have seen big drops.

The Golden Globes held its audience in 2017, but the Emmys saw its lowest viewership ever, with 11.3 million. The 2016 VMAs drew a paltry 6.5 million viewers, down 34% from 2015's 9.8 million, according to The New York Times.

What's going on?

Tim Nollen at Macquarie has a theory, at least for the NFL.

"With a visible improvement in viewership without competition from election coverage, we think it's clear the ongoing primetime declines are due at least in part to competition from other options, from Netflix/Amazon/Hulu to catch-up viewing of VOD and other content," Nollen wrote in December.

This is backed up by a report by UBS — analysts led by Doug Mitchelson showed that the decline in NFL ratings was greater among households that subscribed to a subscription video-on-demand service like Netflix, Amazon Prime Video, or Hulu.

The NFL doesn't just have to compete with regular TV anymore, but also with streaming services that can make the traditional experience of watching TV feel bloated.

Though Goodell doesn't mention competition with streaming services, he said he thinks there are structural changes the NFL can make to boost viewership.

"We want to take as much what we call 'dead time,' non-action, out of the game so that we can make the game more exciting," Goodell said in November, according to The Times.

How? One of the big ideas Goodell put forth was decreasing the number of ads and changing up the way they appear.

"In a world where Netflix has no commercials and consumers are used to 15 seconds of pre-roll, is there a better way to do commercials with our broadcast partners?" NFL Media executive Brian Rolapp said in an interview with Broadcasting & Cable in November. He said that running 70 commercials per game might be a turnoff.

Here's a chart from UBS showing the increase in the number of ads per game over time:

The bottom line is that people are getting used to the type of experience offered by streaming services, and the NFL will have to evolve to compete.

"Up until now, the NFL was a juggernaut, steadily growing in popularity, but this year's audience declines and the palpable deterioration in fan interest in the NFL ... should give the NFL and its TV network partners serious pause," the UBS analysts wrote. "With audiences increasingly impatient and distractions only a digital device away, we would suggest that making the NFL games more engaging is crucial."