They have to do something to get more players
Adding support for Linux is a great thing for game developers - especially for desktop/PC game developers. because PC sales are continuing to drop and the game developers need to look to the future and the potential that linux support can provide. With more and more devices running forms of Linux, adding support now, and building on that support in future, will provide huge potential for where and how games can be played.
For starters, game developers only have to look towards Steam's and the direction they have taken with SteamOS - with the amount of support for Steam within the PC gaming community, and the HUGE support for console gaming already, the SteamOS platform will definitely take off in future, and to be on that bandwagon will provide a great financial benefit to PC game developers as it SteamOS consoles/PCs take off.