Theres a few simple reasons that this steam machine is better:
Game support: Proton plus significantly more native games
Linux is significantly more mature than it was a decade ago (wayland + pipewire + advances in KDE)
The controller is significantly less opinionated (full dpad plus two analog sticks)
The Steam ecosystem and PC gaming as a whole is significantly more mature than it was a decade ago (more games support controllers, steaminput has advanced, and PC game ports are for the most part better than they were during 2015)
This machine actually ships SteamOS and is made by Valve, other companies will make their own Steam Machines later on but this one is the gold standard set by Valve. In this sense Valve has the capacity to demand quality from OEMs rather than OEMs being able to make any demands out of Valve like before.
Quite frankly as bad as the Xbox One/PS4 were they had significantly more games for a lot less money than the Xbox Series or the PS5, competition is simply weaker
Theres a few simple reasons that this steam machine is better: