Why do we say It? Mad as a Hatter...

Why didnt we say that he is "completely crazy" or "strange" or even "eccentric."

Lewis Carroll created the character "Mad Hatter" in his book Alice in Wonderland. The expression "mad as a Hatter" comes from the 1800's, but it is thought to be an expression resulting from behavior after an adder snake bite---more than that related to the character in Carroll's book. "Mad as an atter" has transformed into "Mad as a Hatter."

I dont think anyone really knows...