Well previously it was the fact that it was very AMD centric. Linux can easily support anything of course. But for something that you just want to install and work. Steam OS was made with AMD in mind. You could get Nvidia to work with enough faphing about. But it was discouraged and not default. Enabling it by default could theoretically be less work for Valve. And a value add for the software as a whole.

Before this, said YouTubers were right. It wasn't designed for General use and would cause a lot of problems for people just trying to get into it. Valve changing their support absolutely will change all of this. But it was never guaranteed.