Thomas Dalton wrote:
I really don't know why the main page was put in the article namespace in the first place
I think the main page pre-dates the existence of multiple namespaces, so there was no choice when it was created. I don't know why it wasn't moved when the Wikipedia namespace was created. There is no technical reason for it being in the main namespace (as evidenced by dewiki having it elsewhere), so it's purely a community decision.
And, just to clarify, it can be changed by local administrators: just edit [[MediaWiki:Mainpage]] and change it to point to whatever title you want to be the main page.