There are a ton of developers that will choose whatever language they have available and if developers are switching to mac they wont be reaching for C#. Then you have those that have moved on to F#. Objective-C has the same problem for those that have moved to swift. It is weird to think that next year perhaps 1/3 of SO developers will be on OS X. That is such a huge amount.
And in the last decade Linux has absolutely won the server market. It is going to take some time for the clr on Linux project to stop the bleed from projects that moved off c# just to move off of Windows servers.
Looking at the number of responses to that question, there are lots more people taking the survey in the last two years!
2013 - 8042
2014 - 6537
2015 - 21982
2016 - 49397