Two reasons. First, some people don't like the fact they cannot play the game for free - which is incorrect, they can.
Second, users are reluctant to put the private key into the Dragonglass app. Which is understandable - yet we do not store PK in our servers - it stays encrypted in users phone and is needed because the app makes transactions after a successfully completed level.

Those users, who actually played/mined Dragonglass, give 5 stars