It’s a pain to get other app stores to get uptake on Android since Google refuses to let other app stores be distributed via Google Play. So if Steam starts to distribute games for Android, the Steam app would be thrown out from Google Play.
It’s the same reason why the F-droids user base is so small and will never reach the main public. As soon as your app store needs to be installed via a third party web site, you have lost.
Tracking