You have to take the good with the bad with Fitzpatrick....that includes acknowledging it. He has always had an ability to ignite, his issue is that ignition burns out. The Broncos game was a microcosm of his 16 year career. He looked great and with aggressive play calling scored a TD, and then next drive with the game on the line he forced the ball into double coverage when another receiver was wide open and lost the game.The two benchings when Fitzpatrick was able to take a dormant offense and ignite it was significant. Particularly since many considered Fitzpatrick a below average, journeyman not capable of leading an team to a shot at the playoffs. The Broncos and Raider games removed all excuses it was the players around him, and the only justification left was it had to be the Gailey was calling different plays that was the problem in Tua's defense.
Burrow and Herbet's success, particularly Herbert since Miami could have drafted him was also a factor.
There are more reasons, but those are the main ones imo.
Also Fitz was 3-3 with a full healthy team. Those 3 wins were against the 2 worst teams in the league that had a combined 3 wins on the season and a 49ers team with a QB playing on 1 leg and no Bosa. His losses were against good teams, which he has a 16 year track record of sucking against.
Both Burrow and Herbert had a plethera of weapons that were night and day better than anything we had. Our RBs and receivers (besides a hurt Parker) would be 4th string and practice squad players on most teams. One of them (Ford) went from our PS to the Patriots PS and was one of our primary receivers the last few games. That is how bad it was. Also regardless of Herbert's stats with massive weapons around him, Tua beat him head to head with less talent around him.