Most of the time only admins/mods are the ones who could create those challenges and im not aware about diamond ranks could able to do so or I had just simply missed it out.
Its just right that they would set out some minimum for someone to start up challenge if ever they would really be allowing regular users to create one which I could say that
it would be that interesting but of course it should be on moderation.
I am guessing it is like those counter strike pack opening type of websites. It gives you a certain odd structure that you have to follow, if you go above that then it doesn't allow you, famous with "500 useless 1 cent stuff, and one big thousand dollar knife" type of things there. Same goes here I suppose, a challenge that you have to follow certain pack of rules, and if you follow them then you can do it after a certain level, or at least power, and when you are not there then it doesn't allow you to start it.
Challenges were never really my forte, never participated in them, I like to just gamble the way I like and keep doing that, rather not change how I gamble just so I can win something. Funny enough once upon a time in stake, what I did was a challenge apparently, so I could have participated in that but I learned about it afterwards so I was too late.