Because dealing with the users who experience the problems costs more than they make in sales? If every additional Linux sale costs you more than it makes?
There's being a good citizen. There's wanting to offer support to a small set of customers. Then there's losing money. At some point, you just can't afford to service the long tail.
If the issue the user has found is in the driver, then redirecting them there or maybe sending an automatic email for each unique issue to the driver mailing list might help fix the issue without much effort on the game dev side.
There's being a good citizen. There's wanting to offer support to a small set of customers. Then there's losing money. At some point, you just can't afford to service the long tail.