There are many reasons for this, but to start with: It being Linux based is already enough to kill it's viability.
"Oh but it can run 6000 PC games bro and it's always getting better!"
When people who use Linux say it can "run" something, they only mean that in the most technical sense of the word. Yes, Linux can "run" a lot of things. But it can't run most games well, and it can't run ANY of them easily.
You might get your favorite game running in some fashion after you spend an entire day tweaking settings, applying patches, installing third party wrappers, and installing dependencies for all that shit, just to get the game to boot.
As for the "it's always getting better" part: When it can run every single game that Windows can run, then maybe it'll be something worth taking seriously. Until then, I don't plan on using software based on promises of future updates. That's the same shit as a game releasing in an unfinished state but promising patches later.
The second reason is that Valve can't design a UI for shit. Steam is legit one of the most painful programs in the world to navigate. Every section of that program besides the library is a fucking nightmare. Different fonts, repeating information, useless information, inconsistent formatting, the whole thing feels slapped together by a bunch of Polish children playing with Dreamweaver. I pray that a day will come where I can play PC games in peace without needing to get raped by Ste*m.
And don't even get me started on Nig Picture Mode™.
Windows might be shit but I'm tired of people pretending like Linux is in any way a viable alternative. It's an OS that's designed for programmers and no one else. Anyone who claims otherwise is coping. And I'll die before I use an entire OS designed by V*lve.
@ignika98
I'm sorry, but what are you even talking about? I have tried MANY games on Steam Deck, and so far, only 1 had issues. Yes, there are some QUADRUPLE A games, that fuck themselves with kernel level DRM, but I couldn't care less about them. Valve did the actually impossible and made Linux gaming a single button process.
Sure, Linux is still one hell of a broken mess, but that comes mostly to it being designed by elitists, who enjoy the feeling of supperiority from unironically making it confusing on purpose, so they can laugh at sub-130IQs for not getting it.
@ignika98
Also, Big picture is not made for keyboard and mouse. It's a niche function, that was created for the 1% of gamers, that don't want to throw away an old laptop, so they make it into a smart TV instead.
And as a 1% feature, it also got 1% attention. I seriously don't get what's so strange about that :D
@ignika98
> Steam forces upon me every time I DARE start it up on a computer that's connected to a TV.
Ok, that's new for me 
For something they put "1% of their attention" into they sure don't want me to stop using it.