There is no mention of why this has happened, but you can imagine players are not happy. The Steam page is filled with angry commenters blaming WB (Warner Brothers), alongside the usual sarcastic remarks.  

If you aren’t familiar with the game, it’s an action-adventure open world game. The story follows Scarecrow, the Arkham Knight, and Deathstroke – who plan to release a toxin over Gotham City on Halloween night and unmask Batman. It has many other villains, like Harley Quinn and the Penguin, alongside other heroes, like Nightwing and Barbara Gordon. 

Though the game has been canceled for Mac and Linux, fans on those  platforms still have the Suicide Squad movie to look forward to, along with a Telltale Batman game coming sometime in 2016.

If you need some Batman in your life right now, though, you can always try to build the Arkham Asylum out of Lego bricks like this guy did.