I think Arch really makes sure stuff are compatible before rolling, my 32bit Void laptop has had Python 3.12 for months, and I get all kind of weird warnings when installing Python packages, while Arch is still on 3.11 (maybe testing is on 3.12 idk)
There's that, and there's also the fact that there's only so many maintainer volunteer manhours. It happens to every distro, it just so hapenned that NixOS was faster this time. Though OpenSuse and NixOS suffer from this a bit less, as they've gone out of their way to automate large parts of their update and testing infrastructure(OpenSuse automates everything, I think?).
Part of that is that NixOS had a flake for Plasma 6 throughout the RC phases. So when it hit release, it was mostly just a case of merging it into nixpkgs proper.
You know, I am fine with it. One of the reasons I am using Tumbleweed is for the additional testing they do, so if they aren’t cool with shipping it yet I can wait.