Pain tolerance to prices, how good the support is, how snappy the app is etc. Within the space of game marketplaces they’re average and that’s because every one of them kind of sucks. If Epic was first to monopolize PC game marketplaces people would be defending them like they defend Valve now because they want all of their games in one place.
Linux gaming was stable before Proton. It was never big but mainstream titles were getting released. These days there’s nothing. Titles could be broken at any moment by a developer and nobody will have any responsibility to fix it. I very much doubt that a for profit company does anything because they “like” something like Linux. They’re there to make money, period.
I’m not saying Valve should port their games to ARM or update them, it’s up to them and they don’t seem to be interested in developing games all that much these days. My point wad that plenty of games run via Rosetta2 fine. Steam doesn’t run fine because essentially it’s a web browser and that’s where you can say that 80 developers might not be enough to support this money printing machine.