Games cost money to make. If a game doesn't have a chance at breaking even, it needs external funding or free labour to get it made. This is the economic reality of how games get made.
That's probably not enough for a competitor to make any money, especially since they could only catch a part of these users.
[0] https://www.gamespot.com/articles/as-rocket-league-turns-one...
Steam heavily supports Linux and in their numbers are 0.67% Linux users: https://store.steampowered.com/hwsurvey/Steam-Hardware-Softw...
It is rumored that Steam uses Linux for leverage in negotiations with Microsoft. Therefore it may have a big strategic value for them even though the numbers are still so low.
However, Bungie started as a Mac shop, and Stardock started as an OS/2 shop. It's possible to make the claim that had more commercial games been available to these relatively small platforms in the 90s, there wouldn't have been enough space for these game companies to emerge.
As I said, I can't prove that. It's quite possible these companies would have been successful anyway.
EDIT: I know the gap is small, less than 1% but it is still a gap. If a competitor could emerge in a small gap like this is a question I cannot answer. There is potential for growth (especially if Microsoft slips up heavily which they almost did) but so far that growth hasn't happened and the future is hard to predict.