Audio has been around since, I suppose, the spoken language. The difference now is distribution and availability. Audiobooks, podcasts, smart phones, voice controlled home speaker systems, connected cars, and who knows what will come next.
This is where things can get exciting with your book as a podcast as opposed to (or in addition to) an audiobook.
In a nutshell, the answer to Why Now? is because we can. Sure, microphones and book narrators have been around a long time. But...
Don't know where to price your audiobooks? Here you go.