Why can't I find the official Github mobile app on the Google Play store?

Github officially use a web interface:

• https://github.com/blog/1559-github-s-on-your-phone
• Wayback link

Update (Q1 2020): GitHub have released a new app out of beta testing. See: github.com/mobile/

You can find it here : https://github.com/forkhubs/android

Or this one : https://play.google.com/store/apps/details?id=com.alorma.github