And possibly codding knowledge.
Not all games are ported to Android, because not all games can be ported to it.
The Android version of Ren'Py only offer partial supports of the engine features. This mean that, depending of what the game is doing, some parts can just not works at all. And like the documentation is unclear regarding what is, or not, supported, you've to play the ported game to know if something is wrong or not. With experience you can probably just look at the code to know what will works or not.
There's also possible issue with the UI, that can be too dense to be effectively usable as it on a smartphone. As well as the possible issues with keyboard inputs.
Finally, some games use mechanisms that can't be ported as it on Android. There's some by example that have a mini game relying on keyboard input. They have to be tweaked first in order to still be playable on Android.
Now, all this being said, 80% of Ren'Py games are really basic and don't have an UI, therefore they can be ported without problems.