Political statement? Ease of use? Social experiment? Ideological preference? Spending crypto you were paid in? A stand in until more merchants accept Monero directly? For no reason, but it's not hurting anybody and it's not illegal so why not?
Pick any of the above.
I disagree with this post being downvoted. Manjaro has had a number of issues, including forgetting to renew a cert a few times, accidentlly Ddosing Arch, holding back repo updates but not AUR updates breaking systems, and some allegations of missused funds.
If you're searching for something, I would also personally reccomend against Manjaro, simpy for the reason that you are less likely to wind up with something broken on most other distros. I do know some people who swear by Manjaro though, and if you're using it or set on it then that's fine too (the best OS is the one that brings you the most value).
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To acutally answer the question above, though, the best distro is the one that you prefer. Platforms like Steam manages it's own updates and software so the stable/rolling debate doesn't really apply here. Same with anything installed with distro agnostic package managers (Flatpak, Snap, Appimages). As far as most gaming setups drivers are the only real difference between distros (and you can always change that yourself manually).