What enabled the government's shift in environmental policy and tha passage of meanigful environmental legislation in the 1960's?

There are quite a few things that you could cite that allowed this to happen.

One of these would be the publication of "Silent Spring" by Rachel Carson in 1962.  This book helped to galvanize opinion on the subject of pesticides and the environment in general.

I would argue that rising prosperity helped to allow this to happen as well.  As Americans got richer, they were able to think more about things other than "pocketbook issues."

Finally, I would say the general anti-business, anti-establishment ethos of the '60s helped to push government towards environmental reform.