Of course there's a reason, it's called market share. The monthly Steam hardware survey for April just came out, Windows 10 lost a tiny bit, Windows 7 was up a close to .9% a point. Linux as been well under a 1% for months now. Windows overall grew to well over 96% of all Steam users. If that number is even close to being accurate and I know Linux folks will debate that it's not, but even is it's reasonably close, there's just not a lot of folks PC gaming on much besides Windows and there's just nothing compelling enough about gaming on Linux to make the move. If Linux had a lot more games, better performance (not blaming Linux, that situation is market share related) or say VR gaming was happening on Linux. I would have setup Linux just to do that.There's no reason for League of Legends to not have a proper Linux port, or World of Warcraft, or OverWatch, and etc.
So whatever the issues are with Windows or Windows 10 specifically, when it comes to gaming the alternatives have issues with support that just don't make it worth while for most for gaming. You do see a lot of people around places like this that say they use Windows only for gaming.