This is disabled by default because it doesn't entirely work. WebAuthn is fully implemented in Firefox, and on by default, but U2F is so far still much more common, and the U2F enabled by this feature switch is only kinda-sorta compatible.

Sites need to move to WebAuthn, which works with the same tokens and browsers (well, Chrome, Firefox, Edge) have either shipped or demo'd with a promise to ship. But right now today U2F works in a lot of places if you have Chrome whereas WebAuthn is mostly tech demos. The most notable website that has WebAuthn working today is Dropbox, seamless in Chrome or Firefox, any mainstream platform (including Linux) and all the tokens I have work. That's what everybody needs to be doing.