For a generation, schoolchildren everywhere memorized the names of the 12 planets: Mercury, Venus, Earth, Ceres, Pallas, Vesta, Juno, Mars, Jupiter, Saturn, Uranus, and Neptune.
The list was stable at 12 for about 40 years, but started growing again in the middle of the century. By 1868 there were 100 named asteroids. Not a single one has people living on it, so making children memorize their names was seen as a waste of time. Teach them about the asteroid belt and then move on to more important things. Likewise with the TNO: teach them about the Kuiper belt and the Oort cloud and then move on to more important things. No need to make them memorize Pluto, Haumea, Makemake, Gonggong, Quaoar, Sedna, and Orcus, nor any of the hundreds of other named TNO.
