I can only think of 2 possible reasons-- (1) the support costs are too high for the number of players (online competitive game, I'm sure there's quite a bit of dev time involved in securing the clients & preventing cheating that is Linux-specific) (2) Valve is supporting Linux, epic is busy feuding with valve, and this is similar to a petty toddler breaking their toy so they don't have to share.
Could be either, but I don't see how market penetration of windows in Asia could explain it