Probably because it's really hard/time and resource consuming/tricky to do what ReactOS is doing. By allowing Steam to Linux, they open a new and easy market f

Probably because it's really hard/time and resource consuming/tricky to do what ReactOS is doing. By allowing Steam to Linux, they open a new and easy market for developers distributing games through Steam, where developing ReactOS would only allow their Windows users to move to an Open-Source OS. Porting to Linux makes business sense, investing in ReactOS not so much (at least for Valve).