I was wondering why the rules say to play in Casual difficulty only, and not in any other type of difficulty.

I'm not sure if I updated the rules or if that description predates my submissions, but there is no reason a run shouldn't be accepted under a higher difficulty. The main reason it's played on casual is the grenade count. You have fewer grenades on the harder difficulties, making many of the strategies impossible to perform. Otherwise, you're stuck shooting every enemy you see or blindly praying they miss.

Ah, okay thanks then. In that case, the rules should be updated, but that's not really up to me to decide.