As the title suggests I would like to understand why static classes can have only static members. I hope it is same in all the programming languages. So the explanation would be the same for all the languages, I believe.
Static classes cannot be instantiated, meaning they do not have any instances. Non-static members require an instance of their class to be accessed. Since static classes cannot have instances, you cannot access any non-static members if they exist.
Therefore, static classes can only contain static members.