Pulling support for a client of a competitive online game like Rocket League is fundamentally closing the game to those players - as they will be unable to compete online with other players (be it on PC or on the other platforms).
I really don't like Epic's change in behaviour here - It's quite disheartening since when they used to release first party Linux builds of their products ('Unreal Tournament 2004', and the recently cancelled upcoming 'Unreal Tournament' had Linux client releases), and they were one of the few big name game dev houses to actively support Linux clients.
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