It was likely no secret to most Linux users who know a bit about distributions but Valve has clarified directly that the main reason for dumping Debian Linux for Arch Linux was for faster updates.
Previous versions of SteamOS were based on Debian which has a fresh release every 2 years or so, where during that time most of the software stack is frozen in place. For a Linux gaming device, that's obviously not ideal. Gaming on Linux as a whole often needs more up to date packages because everything moves so quickly. Especially for Steam Play Proton, which has at multiple times needed updates to various packages and newer GPU drivers. Arch Linux on the other hand rolls over constantly with updates and so it gives Valve the flexibility they're needing to more easily pull them in.
PC Gamer, one of the lucky few who recently went to the Valve HQ spoke to Valve designer, Lawrence Yang:
"So, Arch Linux, one of the main reasons, there's a couple, but the main reason is the rolling updates of Arch allows us to have more rapid development for SteamOS 3.0," says Yang. "We were making a bunch of updates and changes to specifically make sure that things work well for Steam deck, and Arch just ended up being a better choice for them."
Valve upgrades the Steam Client constantly and no doubt they will be doing the same with SteamOS 3 once the Steam Deck actually rolls out. Having finer control over everything that they would get with Arch Linux is basically a no-brainer, as is the huge availability of software that comes with Arch and the AUR (Arch User Repository), something that will be a big boon for the desktop mode.
It's not likely that SteamOS 3 will just plainly update directly from Arch though, as that could end up messy. They will likely bundle updates together once they've been firmly tested. More like a Manjaro approach but with more clear QA done.