Niantic claim that, by removing support for Android 6, they will be able to streamline their development process and focus resources on supporting newer operating systems and technologies.
- Sources
Discontinued Support for Android 6 — Pokémon GO Help Center
Pokémon GO ended support for devices running on Android 6, “Marshmallow,” as of release version 241. We encourage Trainers to update their OSniantic.helpshift.com