R0ach.  The reason is because the way the contract is setup before any proposals are voted on pre-investors can withdraw 100% of their originally invested Ethereum (assuming they purchased in the first 14 days).  After the first 14 days the price goes up by 5% a day until it's at 150% (the additional price bumps can't be withdrawn).

I suspect it's people gambling on supply and demand - "If someone is willing to pay 150% on day 31 then I'll be able to sell it on open market for 150%.  Also it will never be worth less than what I paid for it in Eth because I can just cash it out (until crowdfunding starts using the money to fund stuff).

I'm sure there are some hardcore believers.  But only a fraction of the people who purchased DAO.  

P.S.  They just extended the presale by one day.  If you wanted to short Ethereum now's the time to do it.