Once you've made a game engine with the ability to port, it's easier to port a second time. Steam is (was?) just a store, nothing stopped people from porting t

Once you've made a game engine with the ability to port, it's easier to port a second time. Steam is (was?) just a store, nothing stopped people from porting to mac before steam went to it, and little more incentive was there after, especially initially. However now Valve has effectively opened up the Linux market in addition to the mac market. Additionally indie games have historically already had high multiplatform support. Now porting to linux will have the same incentive as porting to consoles; While the game engines will have to move first (Unity recently got there), I give it 5 years before making games for all PC platforms is common practice.