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.
In a nutshell, the answer to Why Now? is because we can. Sure, microphones and book narrators have been around a long time. But...
MK Williams is a wealth of information when it comes to selling more books more easily through libraries. Did you catch that? Sell more...
The ultimate in content repurposing.