Text is cheaper, yet Twitter.

Text is cheap to scale on Twitter, which is why it succeeded. Your message can be read and understood in a matter of seconds. That's much faster than listening to and processing an audio clip.

Text tweets have infinite reach with enough retweets. It can be embedded on websites, drawing a new audience that doesn't spend time on Twitter. It doesn't disappear if you weren't there right when the tweet was being posted.

Audio has none of these things. Nobody wants to watch a recorded stream, the whole appeal is that you are LIVE with everyone at the same time.