Because there have several discoveries of other large objects in space also orbiting the sun and at least one that is larger than Pluto. All of these objects including Pluto are not in the ecliptic (the orbiting plane that all the other planet obit in). Therefore you would have to call them planets too. The leading scientists came up with criteria that to classify a planet and Pluto did not meet that criteria. Pluto also has a highly elliptical orbit too resembling closer to a comet rather to a planet. I think it was a classification well deserved. I hoped this helped!
Pluto does not dominate it's neighborhood. Charon, it's large moon, is only about half the size of Pluto, while all the true planets are far larger than their moons. It's also to far from the Sun. Pluto is now a dwarf planet, there are over 40 of them.