The Google proposal was more full featured than the Mozilla Audio Data API. It provided more functionality baked in vs having to implement it in JS. This was considered important for mobile devices.
Another advantage of the Google proposal was they had a spec and implementation from the start. The Mozilla proposal had a wiki page write up and an implementation and I don't think there was as much effort put into promoting it as Google did into theirs.
Mozilla countered with another proposal, the Stream Processing API (https://dvcs.w3.org/hg/audio/raw-file/tip/streams/StreamProc...) but this was a bit too late.
Another advantage of the Google proposal was they had a spec and implementation from the start. The Mozilla proposal had a wiki page write up and an implementation and I don't think there was as much effort put into promoting it as Google did into theirs.