I think there is still a problem of "not enough developers supporting Linux" - that issue is not completely gone yet, while it has massively improved. The next step is having a stronger adoption from devs + first-citizen kind of performance. But I agree with your sentiment, the improvement has been massive in the past 2 years.
Well ... with the incoming low level apis support on linux should be easier. You need only disk access, network sockets, sound and audio system to make a game. First two are solved problems multiplaform-wise. Remove the drivers from the equations and there is no good reason why porting should take a lot of work.
There are many tools used for Game development that are platform specific and have no Linux version. That's also why some porting efforts are stalled currently.
