Great questions Ian. I thought about it when I saw this yesterday and just couldn't think clearly about it. I think its because the distinctions between street, documentary, reportage, can just be so fuzzy in that a photo taken with the purpose of one of these categories, ie, a "news' photo, or a street photo, can be easily appropriated into documentary.
Practically speaking, "Street and Documentary" share sufficient interests to hang together as a forum. Additionally, you have to ask if there would be sufficient interest to support separate forums for these. A lot of us take photos that fall into either category, and for some of us, street and documentary for that matter can include portraits, various "scapes" and candids, so for me, I don't really see a need to split them