* Six Apart already owns/maintains Typepad and Movable Type. So there may have been some product overlap/redundancy (I believe Vox accounts can be migrated to free Typepad accounts)
* Six Apart has already sold off another of their properties, LiveJournal. So Vox may be another attempt to shed and streamline their overall business. In this case they are closing down the service unlike LJ (maybe they couldn't find a buyer?)
* Tumblr and Posterous as well as Twitter have largely "won" the social blogging war (microblogging in the case of Twitter). I don't think Vox ever had the traction of either of those products and in the economic downturn it may be time to cut their losses.
Just my $.02