They found it because it happened to be where the (faulty) predictions of Planet Nine foreseen a planet to be there.
Look at the time gap between discovery of Pluto (1930) and the next Kuiper Belt dwarf planet (Varuna, 2000). The time gap is so large because people stopped looking for the "ninth planet" once the "Neptune's orbit necessitating ninth planet" theory was disproven. Yes, it was dumb luck that Pluto happened to be where the faulty theory predicted a planet to exist. Still, it was that theory that motivated the search resulting in the discovery. Columbus would have never discovered America if he didn't try to find the western route to India either.