Because its not even beta quality yet, much less ready for Joe Public? for every game you have that "just works" in wine you have a good hundred that MIGHT work in wine IF you have the ability to "massage" the game. this massaging often involves a shitload of hoops and CLI, and Joe Public? Is lucky if he can make a shortcut, he sure as hell isn't messing with CLI.
This is the problem that FOSS advocates simply refuse to accept and I have a feeling valve is gonna find out the hard way....just because YOU think something isn't complex does NOT mean that Joe Public will feel the same!
Until you can remove 100% of the CLI mess from wine, in fact until you can remove damned near 100% CLI mess from SteamOS? Its not gonna fly as the first time Joe Public is told with a straight face "Open up bash and type" its over, they are going back to Windows and furthermore are gonna tell a dozen people how bad SteamOS sucks. And before anybody brings up Powershell don't waste my time, its a SERVER TECH and is NOT used on the desktop. in fact I dare you to show me a single OEM of Windows Home that comes with PS. You could remove CLI from windows tomorrow and the vast majority? would never know it was gone, in fact around 2006 HP had a desktop whose image had no "run" nor did it have any way to access the CMD unless you knew the path...ever hear about it? probably not as it was a home unit and the only guys that ever noticed were guys like me that had to work on them!
While I appreciate the work the Wine guys have done they have a loooong way to go before Wine is simple enough for Joe public, for Joe to use it he has to be able to stick in the disc, go "clicky clicky, next next next" and have it work EVERY TIME, or at least work enough that the few times it don't he thinks its his fault. Wine just isn't anywhere close to that point, sorry.