It was quite a while ago but IIRC it was around the time of a major iOS update (which I had done) and so in the days that followed many of the apps “required” an update which I couldn’t complete. I wish I could remember which app needed updating that finally pushed me to add my credit card as a payment method. But here I am two+ years later and that AmEx is still the card associated with my AppleID rather than my debit card.