It is, because in some older payment systems all payments go via the same "rails" as they call it.

Specifically, the reason that payments are still tied to the nightly cycle is because orders submitted to the central bank aren't executed immediately. At night they suspend submission of new orders and do something called netout or payment compression on the transfer graph, the goal of which is to reduce the amount of money banks need to hold at the central bank. Every payment no matter how small can contribute to the netout, reducing liquidity needs at the CB.