I suspect it's because investors think that one of the few routes to 'exit' a company and cash out your investment just got closed down, at least in the short term. That increases the risk of investing and therefore lowers the valuation.
Also check out Fred Wilsons response (http://www.avc.com/a_vc/2012/06/some-perspective.html). He makes some good points about the returns still being pretty damn good.
Poor market sentiment also affects M&A. I remember from my banking days in 2008, where one of my clients with $2 billion in cash wouldn't even fork over $100 million to acquire a company at 9x revenue with 80% revenue growth.
So over all exit options are appearing less attractive for VCs, thus they're likely to fund less companies at lower valuations.