In the eyes of conservatives, kneeling during the anthem was disrespectful to the flag and to the country, regardless of the motive. The objections were separate from the debate about the prevalence of police brutality and racism — although kneeling defenders no doubt had their suspicions. The point was that the national anthem was a moment for all to rise above particular divisions and stand united in pride and appreciation for the blessing to live in this country.
What was probably under-appreciated by the critics of the kneeling players was that they were willing to take the criticism because they felt that paying that price was worth it in order to call more attention to a problem they thought was being ignored or downplayed. We can argue whether that price was all that high for anyone beyond Kaepernick, as most players continued their careers normally, whether they took a knee or not. But when the players made the decision to take a knee, they didn’t know that their careers were more likely to be derailed by anterior cruciate ligaments than by controversy. For all they knew at the time, they could end up fined by the league or team, suspended, benched, losing endorsement deals, and/or seeing their playing careers end prematurely.