The simple answer to the question posed by the title is, of course, that the defense of the Seattle Seahawks was bad in 2021, and that it was bad in 2020 and 2019 and so on. There will be, of course, those who counter that the defense wasn’t actually that bad in 2021, and that coordinator for the 11th ranked scoring defense should not have been let go. That, however, does not take into account several factors that likely came into play.
Perhaps the most important factor is that in spite of fielding that 11th ranked scoring defense, not once in four seasons did the Seattle defense allow fewer points per game under Ken Norton Jr than it did in any season under Kris Richard or Dan Quinn.