I think the bigger question here is, why the H are hearing aids so expensive????? Does the technology change so frequently? Are the materials so costly? I'm aware they're small, but is the labor so specialized and intensive to justify a price upwards of $4,000????????????
Aside from that, opinion
only: Why are wigs covered but hearing aids not? Well, there are alternatives to compensate for hearing loss.
Sometimes, with total loss, cochlear implants work. There's lip-reading, ASL/SEE, there's (as pointed out above) closed-captioning, there's pencil and paper, there's patience, there's adapting... all if one can't or won't use hearing aids.
Sure, there are alternatives to medically-related hair loss: hats/scarves, or the currently more socially acceptable bald woman. Think, though (and I realize there are a few men on this thread) how important your hair is to you - how much time you spend on it, styling, maybe coloring, keeping it looking good... what a huge effect it has on your overall appearance and, frankly, opinion you have of yourself and how you feel others consider you. Think how you judge
other women when they're having a bad hair day, or simply don't have a lot of hair (
those women, by the way, the ones with thinning hair, don't get wigs covered by insurance).
NOT defending insurance coverage, simply trying to explain why one specific item is covered while another isn't.
By the way, NOT to make this political, but, well, go ahead and try that route. My state representative
claims 
she was instrumental in getting insurance companies in Massachusetts to cover prosthetic devices.