I'm no expert on this so this is pure speculation, but what immediately jumps out from this document is the amount of security- and availability-critical work t

I'm no expert on this so this is pure speculation, but what immediately jumps out from this document is the amount of security- and availability-critical work that depends on each member bank. With wire transfers this has historically been achieved by gating transfers behind physically visiting a branch (not universally true). The synchronous nature (recipient must confirm transaction in real-time) of transfers may also cause annoying failures when recipient banks do maintenance or something at inconvenient times.