It's the Pareto principle doing its thing. 80% of games were fixable by not a whole lot of fixes to Wine (I mean, it's still a lot of work, but once the work is done, you don't need to redo it for 1500 other games), while the remaining 20% are out there doing weird stuff and needs manual fixes of some kind.

If you don't do anything weird, you land in that 80% and everything works as it should. With developers noticing SteamOS being a thing, more of them start doing sanity checks to make sure it works on Linux, and that 80% starts growing to 90%.

Then there's the kernel anti-cheat that's unfixable though, which pulls the percentage down again.