The basic problem is that the definition of a planet has transitioned over time, a lot of time, from a colloquial definition to a scientific one. For the longest time "planet" was just a name for any large, permanent part of the solar system that was worth memorizing. Initially even the Sun and Earth's moon were planets, but the heliocentric revolution placed the Sun in a different category and inadvertently relegated the moon to a minor body along with all the other moons (notable only due to its proximity to human civilization). For a few decades the first 4 discivered asteroids were planets, but the discovery of hundreds and then thousands of similar asteroids brought the realization that these 4 bodies were not quite as notable as once thought. Pluto lasted much longer as the only known KBO for the better part of a century and thus snuck into planet hood.

When Pluto was alone it was much easier to ignore its differences and just chalk it up as the quirkiest member of the planets despite its tiny size, eccentric orbit tilted to the ecliptic, etc.

As we've discovered more planets in other systems and as we've discovered more objects in our outer solar sytem a firmer definition of planet has become more necessary. The new criteria for planet hood are straightforward and useful in understanding the nature and evolution of planetary systems. However, Pluto doesn't satisfy them. In scientific terms this isn't a big deal, but for the public and for the colloquial definition of planet there is still a lot of resistance to change.