Why is this? Do we need to do something about this?
Tags aren't created in isolation, they don't exist until they are first added to a question. So, the simple answer is that we don't have a [left-wing] tag because no one thought their question would benefit from it.
Do you have example questions where you feel the tag would be appropriate? If so, please share, adding a tag to them is trivial.
I'm stealing the answer from Carpetsmoker's comment - it's because "left-wing" is a bad tag (too broad and too vague). So's "right-wing", for the same reasons, and as such I would personally would be in favor of deleting it.