One is considered "testimonial," revealing something you know. The other is not testimonial, just an observation of something you are; it's the same argument used to justify handwriting samples, blood draws, etc.
For the record, I use the fingerprint, but I can disable it in about five seconds. I also use an app (I got mine from the ACLU) that runs a voice recorder and stores the recording remotely, out of the reach of law enforcement. Too many "malfunctions" when they can exercise control over the device.
Thanks. Going to have some reading to do on the app.