I understand Android still has a bit of fragmentation but that number is diminishing greatly as time goes on. I've recently changed phones from Pixel to the Galaxy S20 FE. My phone is of the S20 line but doesn't natively support playing Stadia, as per the list of compatible phones. Is there a timeline or plan for Stadia to be natively supported based upon the Android version and not wait for it to be added to a list of compatible devices? Yes, I personally understand to turn on the feature in experiments to play on my phone but I ask because the average consumer would ask "Why doesn't it just work? I don't want to experiment on my phone?" Etc etc.
I have no idea on which priorities the Stadia team works, but their plan was still to make Stadia available on as many phones as possible. Which phones exactly they have a look at first, I have no idea, but as you can see on the list of compatible phones there are a lot of Samsung phones as well, so might be that they are checking those out as well.
No idea though when more phones will be supported, but I assume they are still working on it. In the meantime you can only be patient or use that experimental mode for the moment.