For some reason I think it will have been even more mundane. It is quite a long time ago, but Matt Levine wrote at the time that GS was (for whatever reason) desperate to get into consumer finance. They saw the market as an open opportunity, and paired up with Apple to get a foothold reaching well beyond their Marcus brand.
[Still paraphrasing Levine.] As one would expect, that didn't go well. Goldman Sachs as a company is geared to do complex low-volume, high-margin deals. Consumer banking by its very nature is high-volume, low-margin. To make things "worse", consumer finance is also very heavily regulated to discourage routinely fleecing your customers.
Which is how you get a vampire squid squad trying to feed off of a decaffeinated strawberry juice carton. Someone is going to be disappointed, and it's not the juice carton.
GS were probably willing (initially) to pay a hefty sum to get into that market, so then the question becomes: why Apple? At the time Apple had a net float of >$200B. When you have that much cash and assets to deal with, you no longer seek the help of a bank. You are a bank.
In a funny twist, their Marcus brand is still alive at least in the UK, and they are offering some of the (supposedly)[ß] best front line savings rates to attract customers. I've never seen anyone with their brand of card, though, so clearly their offered rates are not attractive enough.
ß: few other UK high street banks are offering even better rates, but every single one of them has set a ludicrously low cap on the amount they pay good rates on before dropping to just-about-tolerable rates for whatever goes above the threshold.