He tried lobbying for sensible copyright legislation. The problem was campaign finance. Hence the pivot.
And, you know. Even if Citizens United is overturned (hooray! It'll once again be illegal to make a movie critical of Hillary Clinton during an election! Joyous day!) Lessig's contribution to that is zero; that's going wherever it's going with or without him. It would have been better for all of us if he'd stuck with copyright where he could have made a difference.
Nothing and nobody will stop you from bringing oxygen tanks on your climb, and learning to use them is relatively straightforward. Changing the system of campaign finance in the United States, on the other hand, requires navigating dozens of immensely complex special interests, will be opposed by one or both major parties, and we haven't even gotten to what the courts might say yet. It's hard.