The Demon Lords' strength is that it redirects all battle damage to the opponent, whether it is attacking [i]or attacked[/i]. By negating an attack that targets the Demon Lord, that means I am actually protecting your Life Points in order to take down Life Points another way. So, the attack negation is actually a cost in the case of the Demon Lords. Plus, it's not too helpful if you only have one monster, since Demon Lord LV10's most powerful ability is that it can attack all your monsters once each. There really wouldn't be any point in using it if you only had one monster. In fact, it would actually be worse than something like Magic Cylinder since you could easily use a card that negates my Demon Lord's effect or negate "Counterattack" itself. Basically there are more ways to stop it than other cards.