Monday morning quarterbacks had as much to say about the National Football League’s ratings as the passing and rushing yards of their favorite teams.

The season opener Thursday pitting the electrifying Patrick Mahomes and his Kansas City Chiefs against the Houston Texans should have been hugely popular for a sports-starved nation. But viewership was down by 12% from last year’s opener.

Many blamed social activism. Some in the limited, socially distanced crowd in Missouri booed during a “moment of silence,” yet they were still in attendance. And while the Black Lives Matter movement may not be popular with certain segments of the NFL’s fan base, past threats to tune out games have proven hollow. A 2016 Rasmussen poll found 32% of respondents said they were less likely to watch the NFL on TV because of political demonstrations. Ratings did fall by 8% that year, but mainly during the presidential election campaign, which drew viewers to cable news.

A more likely culprit for Thursday’s light audience was the fact that both the NBA and NHL playoffs, normally over by June, were on television that night, in addition to baseball, college football and the U.S. Open semifinals. Another Covid-19 effect, the lack of an NFL preseason, sapped hype. Finally, unlike last year’s opener, the game quickly became lopsided, with the Chiefs up 31-7. It still was the most watched sporting event since the Super Bowl, and digital viewership broke records.

For sports fans of all political stripes, what really seems to matter is how exciting the action is compared to what else is on TV.

Write to Spencer Jakab at spencer.jakab@wsj.com