This has been discussed extensively. Different systems have different codecs installed, so having the browser pass through to OS/hardware codecs leads to non-interoperability (the same problem as with EME). Also, Mozilla and Google were trying for a while to strong-arm the world into adopting VP8 by denying access to other codecs, but that didn't pan out.
MP3 would be standard codec for audio if the only concern was interoperability. It was officially motivated by non-encouraging of usage of patented formats.
