Technically, Necrozma, Solgaleo, Lunala, Cosmoeg, and Cosmog are legendary Pokemon, not Ultra Beasts, as sugggested by the game and the devs. As we learned from the ultra warp ride, the white wormholes, or worlds, contain one Ultra Beast that lives in that environment. So, Poipole comes from Ultra Megalopolis, and we don't know where Blacephalon and Stakataka came from, so, I believe they gave us Polipole as so to be weak and for us to take the time to evolve it, so like the Magikarp of the Ultra Beasts, and that is all thanks to the dev's choices.
So, essentially, I think Polipole evolves because the devs wanted it to.
