If the media wasn't liberal, why didn't they name those violent left wing groups in the story?

They know all about them, but that info hurts liberals and their causes, so they unethically held it.

The media never has a problem properly identifying violent groups on the far right, in fact lately they routinely try to link all people on the right- especially Trump supporters- to the KKK.

Normally, when I hear people saying the media doesn't lean left, I tell them either they are not paying attention, or they're so far to the wacky left they don't know what right is.

I think in your case you just haven't been paying attention.