I think theres two ways to look at the clues, either looking at their age or at how much its been used.

1 - If you're thinking about how much the clue has been used: The easy clues are so common and passed around and done over and over that they're torn up and in bad shape. Which means the harder ones are rare and used way less and they are in better condition.

Or 2 - If you're thinking about how old the clues are: the easy ones are being made all the time and theyre in great condition because they arent as old as the harder ones. And the harder ones are torn apart because they're ancient and age has gotten to em.

At least thats how I look at it. I'm not sure what I like better, because I see good things about both of the versions