Pluto didn't really fit in with the other planets even before the discovery of further dwarf planets, that discovery was just the final nail in its coffin as a "proper" planet: it was the smallest planet by far, smaller even than Earth's moon, it has a highly unusual orbit (not in the ecliptic plane, so elliptical that it's sometimes closer to the Sun than Neptune - https://en.wikipedia.org/wiki/Pluto#Orbit) etc.