I think since Ultimate Metal is a large community that is centered around all types of heavier metal, it would be in a lesser known band's best interest to have the forum there. I've discovered many great bands through UM forums, bands I would have never known about unless other people directed me towards them. Though you can get the same word of mouth from the homepage forum, having like 60 bands' forums all on one server makes communicating faster and easier.
Or maybe it's just that Antii would rather come up with some sick shreds than moderate a forum. lol