Bill Jacobson explains why they are taking it so hard:

There’s a reason the left is freaking the hell out over the Kennedy retirement. The federal district and appeals courts have been the principal vehicles for achieving liberal political gains that could not be gained at the ballot box or through congressional elections. The Supreme Court, usually by a one-vote Kennedy margin (with the exception of Obamacare, where Roberts defected), also has served that role in fewer, but the most important, cases.

We are used to losing institutions. The left is not. They are waking up to the possibility that the judiciary may be restored to the neutral role it should play, and would no longer serve as a liberal super-legislature.