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.