First, because it was thought a much larger planet would be found there, but Neptune's and Uranus' orbits were wrongly calculated and there was no need for a massive planet X to exist.

Then, because the telescopic instruments got better and better , Pluto got smaller and smaller, was even split in 2 worlds (now even 4).

Then because many other objects were found in the area, matching a theory about such objects.

All those discoveries, made by american astronomers, KEPT PLUTO IN THE NEWS!!!
That's why it became a cultural issue.

However, ALL THOSE DISCOVERIES pointed at the same direction: Pluto cannot be considered a planet like the 8 other ones IF it already made sense back in 1846, to demote Ceres, Pallas, Juno and Vesta.

Before this IAU session, there was hardly any talk about renaming Ceres a planet. Ceres was not as mediatized in the US as Pluto had been.
It is therefore correct from the IAU standpoint to call it (Ceres) back IF something was going to be done about designating Pluto, or else Pluto should join Ceres ranks.
The IAU did have more a science logic than a cultural one, but indeed there were mistakes made even in the official painting (shadows):