My theory about the Apple Card:
Goldman Sachs is not setup to be a direct consumer bank. This caused a lot of problems:
* The ease of sign up / approval led to a lot of subprime approvals that might not have gone through with a more established lender. Not only did this cause GS to lose money on delinquent accounts, it prevented the Apple Card from ever becoming a status symbol.
* Goldman Sachs developed a reputation for almost never backing up cardholders in the case of dicey/fraudulent charges.The sense of security is a huge part of why people use a credit card for purchases. I myself terminated my Apple Card account over this.
* The card itself was just not competitive in terms of rewards. The UI/UX of the app and the deeper integration into the Apple ecosystem was nice... (if a bit confusing) but when I moved over to Chase I found myself not really missing it at all.
* Edit: Forgot one additional thing. Apple Card did not have contactless (tap) technology because they wanted to promote using Apple Pay. But whipping out your phone sucks compared to just tapping your card against a reader.
All I wanted was a simple, easy credit card. I didn't care about minmaxing bonuses etc... but when I moved over to Chase I realized I had been leaving a lot of money on the table.
All the other players but especially Chase and AMEX have very established reward systems setup.
It's possible that Apple wants to move banks specifically because they want to reposition the card and need a different partner to do it with.