The popular opinion is that it was easier for them to get up-to-date packages that way.

My opinion: It's just what the people working on the Deck were using at the time themselves.

Other reason might be that they had SteamOS 2 based on Debian and probably had some problems with it that they could solve on Arch more easily.

For KDE, Valve found it easier to work with KDE devs than GNOME devs.

Gaming support is still very much a work in progress all up and down the software stack. Stable distros like Debian tend to ship older proven versions of packages so their packaged software can be up to 18mo behind current releases. The NTSync kernel code that should improve Windows game performance isn't even scheduled for mainline merge until the 6.10 kernel window in a few weeks - that's not likely to be in a stable Debian release for a 12-18mo.

TL;DR: Gaming work is very much ongoing and Arch moves faster than Debian does. Shipping 12-18mo old versions of core software on the Steam deck would degrade performance.

As for why they adopted KDE, they probably discovered how hard it is to work with Gnome developers.

Games need to live closer to the bleeding edge than a lot of other software.

Also, for wine/proton, and the other customisations built into the deck, it makes sense to pick a starting point that is more built for customisation. By that I mean there was probably less things they needed to add or remove at the start.

As mentioned, it's also likely there was personal bias internally. But even that can be a valid reason as they need to be familiar/comfortable with the starting distro.

Not saying that Debian cannot do it, but doing it this way probably made valve's employees lives easier.

It was based on debian, but moved to arch.

I think they did it because honestly, arch is better for desktop-usage due to its rolling-release model.

Bugs in debian stick around forever.

All of the things others have said are excellent points, I would also like to point out that if you go to the steam hardware survey and select Linux only you'll see that Arch is the most used distro (after SteamOS), and that was also the case when the Steam Deck was announced in July 2021 https://web.archive.org/web/20220806051441/https://store.steampowered.com/hwsurvey/

And from my personal experience there's a reason for that, other than the I use Arch btw meme, despite most ports having Ubuntu in mind, and despite Ubuntu being the more user friendly distro, games just work on Arch. It's a weird thing where gaming on all of my arch machines is very painless, but gaming on the Ubuntu ones is frustrating, there's always something not right, it feels like the machine is chugging, or the driver decides not to work, or the game shows a black screen, or prime decides not today, etc, etc. I admit this is personal experience, and others might have the exact opposite, and that this is kind of biased because as a general rule people who use Arch tend to be more knowledgeable about Linux than people who use Ubuntu, but from replying on several Linux forums it's generally people with Ubuntu that have problems with games and people with Arch usually report that "it just works" for them.

the deck isn't some server that needs > 100% uptime for years. Debian is poopoo for bleeding edge game releases, especially any alpha/beta/early access stuff

[...] anyone know why Volvo went in this direction?

So noone is talking about Volvo?

Other than that, SteamOS started with Debian and switched to Arch last minute before the steam deck released.

Rolling release, quicker updates for gaming, and pacman is an extremely fast package manager, which is why OpenSUSE Tumbleweed wasn't chosen. KDE probably because touch screen works better on it and maybe they found switching between desktop and big picture mode to be a better transition

In early Steam Deck showcase videos there were talks with Valve guys like Lawrence Yang, and IIRC they simply said that it is easier for them to build the system that way, not that they couldn’t continue using Debian.

I think the reason for that might be that Debian has pretty strict package and dependency policies and sometimes it’s not easy to put cutting edge solutions on top of the „stable” base, so they would end-up using unstable/sid anyway, which still isn’t ideal as there is some freezing happening every now and then. Also Debian packaging system feels quite dated and strict comparing to PKGBUILD format, and it’s simply easier to build custom packages, having single build instruction file is super convenient and unlike with Debian at times, replacing whatever core system packages without breaking half of the dependency tree is usually easily doable on Arch.

Another point for KDE might be that it works much better on a small screen that may be partially obscured by an overlaid keyboard. I used Bazzite Gnome for a while on the Steam Deck and I much preferred Plasma on there after switching back, despite using Gnome on my main system.

For the KDE part, something I haven't heard most people mention is the wayland support and how fast they are to pioneer and implement new protocols. DRM leasing is the reason why Gnome can't do VR games and I forget why they wouldn't implement it, but the why doesn't really matter for a company focused on gaming. There are quite a number of protocols that have followed this same story with Gnome.

Debian was planning on dropping 32 bit support and Steam is still a 32 bit application so valve freaked out.

Arch gets faster driver updates, KDE is faster at developing Wayland protocol implementations.

Edit: Valve gets their desired stability by turning Arch into a point release distro through image based releases. And, the system is practically unbrickable since it's immutable. So, in summary it's the best of both rolling release and point release models. By best, I mean for gaming.

I suspect KDE because most PC gamers are Windows users and KDE is closer to that while Gnome is closer to macOS (both in design and being restrictive).

I believe SteamOS is also immutable and uses a rolling release model. It’s probably logical to make a custom version of Arch. They can make it immutable and still get the latest packages. Fedora Silverblue (or another immutable Linux distro) wouldn’t be as quick to release packages and was probably in alpha when the decision was made.

SteamOS, which is what is on the Deck, used to be Debian-based. The creation of the Deck led to an environment where a rolling distro was a better choice.