Bradley Chambers offers three reasons, only one of which I think is key. In reverse order:
Voice minutes are pretty much unlimited now. My AT&T plan has unlimited nights and weekends and unlimited mobile to mobile (any carrier). I am using an average of 150 minutes a month at this point.
Most Canadian carriers (and carriers in many other countries) offer very few minutes in standard plans because almost nobody uses them. I have 200 minutes on my plan, but very few of those get used in a month.
Most carriers also offer unlimited SMS/MMS, but that didn’t prevent iMessage from gaining substantial market share.
Data plans aren’t unlimited and long FaceTime Audio calls will use a decent amount of data. While it’s nothing like transmitting video, it still averages out to be about 5 MB every for about every 10 minutes of usage. Over time, that can add up to be a decent chunk of your data usage. Even if you have a 5 GB plan, what about the person you are calling (my wife is on a 300 MB plan)?
While it’s true that data plans aren’t unlimited, I’d wager that most people place fairly short phone calls (see the above bullet point regarding how little most people seem to use their voice minutes).
Chambers cited a 150 minute per month average, which works out to about 75 MB of data used per month using his 5 MB per ten minutes figure (I saw a similar amount of data usage) — not insignificant, but not massive. That figure is also assuming you’re placing all of your calls while on a cell connection; how often are you in your home or office where you can offload that data onto WiFi? I’d bet regularly.
This is the clincher. If Apple makes FaceTime Audio a default — even if it’s a preference — I think it will be very successful. I’d use it as my default. But, as it is, I think it will be a niche option for those who find it.