...so they picked a rolling release distro to take snapshots of and distribute outdated versions to their users. Plasma in SteamOS 3.6 (current beta) is still at version 5.27, just like in Debian stable.
The main reason they switched were PKGBUILDs. Debian packaging, while much more powerful, requires more effort, and yet they weren't going to reap its fruits as their distro gets fully reinstalled on each update anyway. Nothing else matters much for their particular use-case. If it was about fresher base system, they could have gone with snapshots of sid instead, which would have been easier as previous SteamOS versions were already Debian-based.
The main reason they switched were PKGBUILDs. Debian packaging, while much more powerful, requires more effort, and yet they weren't going to reap its fruits as their distro gets fully reinstalled on each update anyway. Nothing else matters much for their particular use-case. If it was about fresher base system, they could have gone with snapshots of sid instead, which would have been easier as previous SteamOS versions were already Debian-based.