There are a few reasons why they would go short. As someone said here, most of wall street experts think that Bitcoin won't survive and is not capable of replacing the current financial system. Don't you also think that they could influent the Bitcoin price using Bitcoin futures and contacting someone with a huge number of bitcoins? They could bet that Bitcoin will soon decrease and then dump a lot of bitcoin. Quite simple and profitable, isn't it? I wouldn't be surprised if most of them have already bought Bitcoin some time ago.