Part of it could be them becoming one note characters. Bernie is the squeaky voiced bully and Amy was only written as a desperate and pathetic overly attached girlfriend in season 8.

Another thing is the Tardis incident where Bernadette tries to sell it and Amy dismantles it into a door frame to try and lure Sheldon into sleeping with her. Alot of people where butthurt about this for some reason. As the show becomes less about endless pop culture references and product placements now becoming more about dating and relationships.

Focusing on Amy and commenting as someone with a psychotic devotion to her. I would say alot of it is just the bad writing. Amy is shoehorned into scenes probably as part of Bialik's contract. When she doesn't have anything physical/romantic to do or catty to say her lines are actually painful to hear as they are really unnecessary and probably filler for weak scripts.

The next thing is nostalgia, the show was better before Amy and Bernadette. But not necessarily because of them. Berandette had her moments and Amy added alot of great moments to seasons four and five. Great sitcoms like The Simpsons, Futurama, Frasier and Roseanne went on too long, the writers ran out of ideas and like with peak oil or something they found whatever low quality humor that was left. IT Crowd which ended at the exact right time had a terrible one hour special a few years later to end it.

The show like a terminal patient is dying and it's fading popularity keep alive long enough to become a sweaty mess. (I almost said die with dignity but this is the big bang theory where colonoscopy jokes have been standard since day one).