They've tried! ACH has been somewhat modernized over the decades, in the form of compliance and in-protocol changes (faster turnarounds, additional transaction codes for online transactions and different cardholder verification methods). The Fed has also been working on a modern replacement to ACH for at least a decade, and is planning on performing an initial release in 2023[1].
But as for why it's taken so long: banking in the US is, for various reasons, significantly more complicated than banking in most other countries. The US has thousands of FDIC-insured banks and credit unions, each of which operates under a patchwork of municipal, state, and federal regulations. Banks are (mostly) required to honor each other's checks and transactions which means that, in the worst case, there's a total graph of all ~10,000 banks and credit unions serving as both ODFIs and RDFIs[2]. Transaction failures between any two nodes in that graph are a possible compliance failure, so any common mechanism is both a lowest common denominator and resists any modernization or other changes than can cause disruption.
[2]: It's actually simpler than this, since the ACH clearinghouses serve as a central resolution and dispatch service for all ODFIs and RDFIs. But each O/RDFI still has to interpret the ACH record(s) they receive, so there's still extraordinary inertia against any changes.
But as for why it's taken so long: banking in the US is, for various reasons, significantly more complicated than banking in most other countries. The US has thousands of FDIC-insured banks and credit unions, each of which operates under a patchwork of municipal, state, and federal regulations. Banks are (mostly) required to honor each other's checks and transactions which means that, in the worst case, there's a total graph of all ~10,000 banks and credit unions serving as both ODFIs and RDFIs[2]. Transaction failures between any two nodes in that graph are a possible compliance failure, so any common mechanism is both a lowest common denominator and resists any modernization or other changes than can cause disruption.
[1]: https://www.federalreserve.gov/paymentsystems/fednow_about.h...
[2]: It's actually simpler than this, since the ACH clearinghouses serve as a central resolution and dispatch service for all ODFIs and RDFIs. But each O/RDFI still has to interpret the ACH record(s) they receive, so there's still extraordinary inertia against any changes.