Actually, I opened a second game and there was a "pay by card" option, which worked. So I suppose it's a per-game setting? Scratch all I said above then.
It's not that simple, game authors on itch.io have the option to collect money directly rather than going through the platform, in which case they're free to support alternate payment methods. But that's an option open to few, since compliance with financial regulations is increasingly difficult these days. For example, I pretty much have to let itch.io collect payments for me, and that does indeed limit who can buy. So your request certainly is justified.
I see. Thanks for the clarification. In that case, don't scratch what I said above. On the subject of spending money on here, bought your book! It seems interesting. I do want to get into making my own games so desperately.