Because Firefox is like a democracy, they prioritize work based on number of votes on issues/feature requests. The AudioEncoder API has literally just one vote, and the overall WebCodecs API that it's a part of only has five votes. This shows that there's very little demand for it, meaning very few sites actually use this (that or the vast majority of Firefox users don't use/need this feature). Why bother focusing your efforts on implementing something that most users don't care about? The higher priority things that most Firefox users care about is stuff like performance, and Mozilla have been making some good progress too on that front.

This is an experimental API that hasn't been finalized yet. Firefox devs has limited engineering resource and simply can't keep up with Chrome's push to implement experimental/proposal API. Safari also hasn't implemented this yet because they also usually wait until the API finalized, which can take quite a while.