Could be for many reasons.

Androids aren't exactly fast and maybe they think it's not worth the trouble. Or the Rosetta software would need an update/changes to the code and again, maybe it's not worth the trouble since the benefit probably isn't that great.

I'd like to see a Linux ARM app though as those little ARM devices seem very efficient.

Rosetta@home (from the creators of the Rosetta software, which MIP uses) has both an Android application and a Linux ARM application. Linux ARM devices contribute more than double what the Androids do. Ultimately, neither is a whole lot when compared to Windows/Linux running on Intel/AMD CPUs.

Of note, Rosetta@home runs a significantly more recent release of the Rosetta software compared to what we are running here. And I believe it has been said that the scientists of the MIP project are happy with the current version so we can't just ask Rosetta@home for their code and run it here as that would mean all apps would have to be updated to the more recent Rosetta code and possibly other custom changes would have to be made.

Ultimately, it's up to the project scientists and WCG but I don't think we're getting any Android/Linux ARM app soon. The focus now is on the OPN GPU app, MCM beta, the new climate project, maintaining all projects, etc.