Two reasons:

• Sleep data can give some information on your health but isn't regulated as strictly as health data. Health data == Money
• Companies want your time and attention. TV manufacturers want you to use your TV as much as possible, social media companies want you to use their platform as much as possible, video game developers want you to play their games as much as possible. Nintendo successfully gained people's time when outside through Pokèmon Go already. Now they're trying to expand to your time sleeping. There's not much time left which isn't utilized by companies somewhat. Maybe there will be a Pokèmon School/Work soon?