I'm always confused by the hostility I receive as a Linux gamer. My guess is that people simply feel inadequate. In the Windows world they're used to being feeling technical and superior (overclocking, tweaking, etc). A user posting about Linux makes their lack of technical knowledge painfully obvious. Just that fact that several posters on your review didn't realize that SteamOS is Linux proves my point.

Just because somebody is using Linux does not mean they're criticizing another person's decision to use Windows for gaming.

In addition to the feelings of insecurity, my guess is there are a couple other reasons:

1. They would prefer devs not "waste" time on another platform.
2. Some Linux users can be very elitist and criticize anyone that uses Windows. This probably creates an adversarial environment and exacerbates the feelings of technical inadequacy.