Because he was a bad fucking GM from player eval to salary cap to coaching eval that put together losing teams. If Vic stuck around and the Bears somehow continued to win football games with a good defense and shitty QB play, he would still be GM. It's about the wins and losses not how they happen. Just like in golf, they don't ask how, they just ask how many.
I'm not trying to support Pace, because he wasn't good. But he certainly did well evaluating Montgomery, Kmet, Leno, Jenkins, Whitehair, Roquan, Trevathan, Johnson, Jackson, Robinson, Mack, Hicks, and several other players. Your hypothetical of good defense with bad QB play was never going to happen because that's not a sustainable way to win games in this century. So, it's irrelevant to even imagine.
It ALL comes back to the QB. Nagy got the job because he was around Smith and Mahomes when they played well. He would have looked like a great coach here had the QB been good. Obviously, wins and losses ultimately decide a GM's/HC's fate, but wins and losses are largely determined by the QB. It's why Mahomes didn't suffer by losing Tyreke Hill. It's why the Eagles took the next step to eliteness. It's why the Bengals went from a bad team to a Superbowl contender 2 years straight. Conversely, it's why Washington can't get anywhere. It's why the 49ers spent 3 1sts round picks for a QB because they couldn't get over the hump with a fantastic defense and mediocre QB.
Sure technically, Pace was fired because he didn't win enough. But he didn't win enough because he got the QB position wrong, and tied 4 years to Trubisky who was a failure of a draft pick.