It's all about demographics.
Your enthusiast Nintendo fan - the type that'd care enough to do things like post on a forum - grew up in a time before the internet was mainstream. While they may love Nintendo, they're not all that likely to hang out on websites like this, even if they do keep relatively informed of news via sites like IGN (ie, my brother-in-law - knows almost as much as me about Nintendo, but has never posted on a forum).
The average PlayStation fan, who got into gaming via the PS1/2, grew up alongside the internet. Hanging out online was normal for them, and it's lead to an older generation of gamers that still do this to this day.
The Xbox fanbase hasn't matured enough to really have a large internet presence. It's older fans usually grew up playing Nintendo and Sony systems, so their gaming fandom isn't exclusive to Xbox, while the younger generation that grew up with Halo is rather small and limited primarily to the US/UK.
