Originally Posted by matmosphere
Like the idea of a Mohs hardness scale, not a fan of the execution (and Ohm’s scale of rock would be better). There are more ways for music to be heavy than just distorted guitars.
I think reason it's called a scale of Rock and Metal Hardness is both a play on the Mohs scale of mineral hardness and both terms can refer to solid materials as well as music, and that rock and metal are more likely to be described as "hard" or "heavy" than other forms of music.
Like I said above, some of the higher level examples don't have guitars at all. I think when the page was first created the examples were just rock and metal bands (most of the links are to YouTube videos that are 10 years old or more, and some have even been removed), but it has come to mean a scale of how "listenable" an artist or group's music is for the average person.