There is nothing difficult to grasp. They made rules then decided for no reason to let mercury break the rule. Why? Why not make mercury a dwarf planet instead of allowing it with no rule exception other than...just because.

This is not bioligical... those MUST follow the rules. This was a traditional unscientific list.. Exactly like constellations. Why not start removing stars from constellations because they are too far away? Except a couple of them just because.

This IAU conference vote was not unanimous... it was very contentious and many wanted a more geological and broad definition rather than an earth centered definition that literally ONLY applies to our solar system. "Planets" can only exist around OUR Sun. Think about that.