I understand the exception created for Neptune. But they had to create this exception... for their own brand new rule... in order to classify 8 things. Notice the exception is written very specifically just to keep pluto from "clearing" is orbit.
Another IAU rule is that the body must assume hydrostatic equilibrium(nearly round). Mercury does NOT assume hydrostatic equilibrium. They knew this.
Guess what? They just...decided...Mercury doesn't have to follow that rule.
It was all done very unscientifically.
Edit: I want to add that now there are only 8 planets...in the universe. There are no other planets because the definition includes that they must "orbit the Sun". Not a star but very specifically the Sun. All this with exceptions for just 8 objects? I'm telling you it was a power trip thing more than a scientific endeavor.