This is a pretty well-known effect in gamedev. The specific numbers change, but Linux sales are only a small percentage of total, while generating much more development and support work.

At least dev work can be drastically simplified with middleware like Unity, which is how small studios can even consider Linux. But support difficulties are real. Players have so many possible distros, configurations, and drivers - actually supporting all of them would require a level of Linux expertise that gamedevs simply don't have (being typically Windows or OSX based themselves). Limiting yourself to something like Ubuntu LTS helps a bit, but there are still plenty of gotchas.

So it becomes a simple matter of numbers: given relatively small amount of extra sales, is it worth the extra work, and spending the time to learn Linux development and administration in sufficient depth? Sadly usually it is not.