There are a myriad of factors.
1) The Chicago Bears were overrated.
2) The defense in 2023 played well against backup QBs.
3) The Chicago Bears started training camp with one viable defensive tackle in Andrew Billings. Gervon Dexter came on and seems more viable now so that gives the Bears two viable defensive tackles that they played with all year. You can't be a good team with no defensive tackles.
4) The Chicago Bears had two viable defensive ends to start training camp. Austin Booker is not who you run out there as your third DE. Maybe he'll be good in 3 years.
5) The offensive line was a mess. Injuries also piled up in a massive way.
6) Coaching was awful and probably cost the Bears 3 wins if not more.