Probably because the state has limited ability to compel business relationships, thank God, and taking card effectively means enforcing a business relationship with a duopoly.
It's a common misconception that arises from state legislatures doing nothing for several decades, but the power is there.
For the record I don't support either and believe it's up to every business to decide what form of payments to accept.
I realise it begs the question a little, the state can mandate cash because it's the state's mandated and controlled medium of exchange, but state power often begs the question.
Any business that is large enough can get their gift cards in retail stores which would be fine. It would definitely create a moral hazard if Netflix had to provide a cash option like this because retail stores would suddenly have crazy amounts of leverage but w/e. But then naturally there would be market need for "accept cash payments for your online service" as a service for smaller businesses that can't just get partnerships with Target and Kroger and the entity providing that service would just be a bank issuing branded prepaid cards but using their own bespoke card network.
Like you can't fight it, you always invent Visa.
You could probably cut card networks out eventually with $as_of_yet_unlaunced_service backed by FedNow but that's not exactly cash and still requires a bank account.
No attack on you, but I feel like lots of people are suffering from not only a lack of imagination, but a serious lapse in memory for the days when they didn't always have a card and not every subscription was an online direct debit arrangement.