That's poor production decisions. Twtich is also 720p and very rarely 1080p only when ZP does the streams.
Even in their own website the quality in "Best" is quite bad http://tv.majorleaguegaming.com/

Check this streamer on Youtube with 720p: https://gaming.youtube.com/watch?v=2WhbUkPfTQw
It's amazing how the quality is much better, now imagine in 1080p... Like I said is the MLG who decide to give us shitty quality streams

Youtube has better bitrate to watch Overwatch on stream, Youtube>Twitch to watch Overwatch games