Why was Pluto disqalified?

Answer 1

I assume you mean "as a planet".

The main reason is that the more you study the solar system the more obvious it becomes that Pluto just doesn't deserve the name. Everything else that was considered to be a planet was by far the largest thing anywhere near its orbit.

Earth shares its orbit with its satellite the Moon, about two orders of magnitude less massive, and that's the closest any of the actual planets get to having something else of significant relative mass near their orbit. Pluto's orbit crosses that of Neptune, which is about three orders of magnitude more massive. Neptune is just behind Earth with something in its orbit that's about 2.5 orders of magnitude smaller... not Pluto, but its own moon Triton, which is noticeably larger than Pluto is.

Not only does Pluto's orbit cross Neptune's, it turns out there's a lot of rock that spends most of its time out beyond Neptune, and Pluto is just the biggest chunk we've happened to find so far (and not by much at best; Eris is roughly the same size, and may even be larger).

Lumping Pluto in with the planets when we thought it was the size of Earth made sense.

Calling it a planet when we discovered it was maybe as big as Mars stilll made sense.

Calling it a planet when it became clear that not only was it not as big as Mercury, but not as big as the Moon? Ludicrous.

The IAC definition of "planet" seems a bit arbitrary in some ways, but realistically planets are kind of like pornography: it's hard to define, but we know it when we see it, and Pluto ain't it.