The reason stated in the title of this thread. Usability. "Mark Forums Read" is not necessarily as clear as "Mark all forums as read". Not only does it read better, but it makes more conversational sense.
Then you should make that suggestion at vbulletin.com as it would need to be changed in the standard package.
I don't see a reason to change it only here, especially considering we are all admins (or at least knowledgable users) of vBulletin boards and everyone is used to this term. I strongly doubt it will make our forums more usable for our audience.