Stock trading on exchanges traditionally has delayed settlement. You don’t need to have the money and stock at hand when you make the trade, but you’re expected to provide them on the settlement date. For a long time this was two days later, but is now being changed to only one day in the U.S.
Trading on Saturdays would either mean complications for settlement, or that settlement must happen on Saturday too. Surely not impossible, but these things have ripple effects that are not immediately obvious.
> “Why can't I move money from one bank account to another on Sunday?”
This exists in Europe with 24/7 SEPA instant credit across banks and Eurozone countries. It doesn’t cost anything extra.
But getting to that point needed a serious mandate from the European Union, years of planning, and cooperation by the various governments. The effort was worth it because it’s a far better solution for consumers than e.g. cryptocurrencies or the American proprietary payment solutions like Venmo.