Mulvihill has two overarching explanations, and I would add two more.
He credits the presidential election won by Donald Trump over Hillary Clinton for sucking away some interest in the first half of the season and notes an increased number of viewing windows spread that larger audience thinner.
In that case, there were not fewer people being interested in the NFL – which would be a potential crisis for the league – but more likely those viewers just weren’t terribly enthralled with what they found when they did tune in, especially compared to the twists and turns of the election.
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And you know what? It just so happens the games in a lot of those extra windows – particularly primetime games – were bad matchups, noncompetitive or both, so it stands to reason the overall effect on average viewing time would be affected disproportionately.
Extra windows also mean fewer games in the traditional Sunday afternoon slots, making them more susceptible to lacking competitive games and/or intriguing matchups, too.
Does that mean the NFL can take its popularity for granted? No. The concerns about a lack of marketable stars and pace/quality of play are valid.
They just might not have hurt much… at least not yet.