The simplest explanation is comedy is subjective. Garth Marenghi's Darkplace isn't objectively funny. It's funny to you, it's funny to a lotta people, but it isn't funny to everyone. If it was objectively funny, it'd be funny to everyone.
He can't disagree with you?
Nope; it's because those who, in your opinion, underrate it don't think it's as funny as you do. Don't try coming up with excuses for why people disagree with you; just accept it.