Officially, the Android version of the Pokémon TCG Online is currently for Tablets.
We certainly understand the desire for phone versions of the game. We want to play on our phones too.
The game was originally designed for a robust (wifi or wired) data connection on a PC. The game has not been optimized for use over a limited cellular connection, that players on a smartphone might expect. Also, there are user interface considerations that could be optimized for smaller smartphone screens. For these reasons, any device that Google Play considers a “phone” is not currently enabled to download the game. If Google Play views your device as a "phone" it will come back with "your device is not compatible" and you will not be able to install the app from Google Play.
Those are the official reasons for the game being labeled as tablet only. Hopefully that helps at least understand why it is as it currently is.
Thank you in advance for your understanding.
Pokémon TCG Online
The Pokémon Company International
Need help from the support team? Visit the [url="http://www.pokemon.com/support"]support portal[/url] and submit a ticket!