It needs to involve everyone because the only way to get there is to force them, and forcing only some of banks is kind of unfair.
E.g. the EU got there by legal limits on transaction settlement time set by the first payment services directive, the infrastructure came into place only when the banks got the message that they have to improve the product or else (IIRC the law specified that after a couple years, customers would be entitled to compensation if their payments were "too slow"). Without the requirements, EU banks would still be happy to charge 10 eur for a 3 day payment, since that's obviously more profitable than charging cents or zero for same day payments, even if the payments would be running over fast&cheap infrastructure - case in point, UK banks, which charge extreme fees for EU EUR payments simply because they, as a non-EUR country, aren't forbidden by the EU to do so.
E.g. the EU got there by legal limits on transaction settlement time set by the first payment services directive, the infrastructure came into place only when the banks got the message that they have to improve the product or else (IIRC the law specified that after a couple years, customers would be entitled to compensation if their payments were "too slow"). Without the requirements, EU banks would still be happy to charge 10 eur for a 3 day payment, since that's obviously more profitable than charging cents or zero for same day payments, even if the payments would be running over fast&cheap infrastructure - case in point, UK banks, which charge extreme fees for EU EUR payments simply because they, as a non-EUR country, aren't forbidden by the EU to do so.