This has been suggested a few times as you can see on CC (like JustLeafy's blog). I don't think that there is a reason (I believe there was a way to delete blogs in the past - see here). Best thing to do would be send in a feedback form at Special:Contact and see what staff reply.
Perhaps the reason this isn't allowed is abuse (if you make loads of blogs and delete them quickly, it's clogs up the deletion log and therefore recentchanges). I'm not entirely sure the reason but, if you choose to contact staff, please let me know the answer!
Like I said, I was only taking a guess. It is possible MediaWiki has that ability and Wikia just doesn't use it. However, that being said, it isn't enough to restrict based on namespace alone. You need to make sure that the user is deleting their own blog, not someone else's.
You are right. Noreplyz's reply does not answer your actual question. However, I am guessing he posted it because we sometimes get people asking "How come x can't do y?" when what they really care about is "How can x do y?".