No reason, really. I’m not part of the “hate on Manjaro” club.
I got started with Manjaro because I was looking for an Arch-like experience, but with better distro management, curated packages, etc. I’ve had some of my best PC gaming experiences on Manjaro with Luttis and Proton, it is a great Linux gaming distro.The distro managers have definitely let me down more than once, most notably when they wouldn’t ship KDE Plasma 5.25 when it was released citing “stability concerns”, and then doing the same thing with Plasma 5.27. But those issues are behind us, and didn’t affect me too badly (I just needed to wait 6 weeks until the next release to get my updates). I’ve come to realise through my use of Manjaro that I actually always want to use it like Arch. Often things I want to install are not available in the Manjaro repos, but are available on AUR. Then installing from AUR sometimes depends on things that are not in Manjaro repos. It gets messy, and I should just use Arch.
But rather than moving to Arch, I think I am itching to move to something completely different, and NixOS and Void are about as different as it gets.