Why members of interface cannot be static?

All static methods in .NET are non-virtual i.e they can't be overridden in derived classes. 

This means that if interfaces were allowed to include static members, then the implementation of those members would need to be included in the interface itself and that, if a class which implemented the interface had a member of the same name, then it would be entirely separate from and wouldn't override the interface member.

Some .NET languages do in fact allow static members in interfaces even though it's not CLS compliant.

For example in C++/CLI you can do this:

interface class IFace

{

   static int i = 3;

   static void Print() 

   { 

     Console::WriteLine(i);

   }

};

However, as you'll observe, the implementation of the static method has had to be included in the interface itself and can't be overridden in a class which implements the interface. You'd simply call it like this:

IFace:Print();

The designers of C# and VB.NET evidently thought that this was pointless and made interfaces more complicated than they needed to be. You could just as easily have included the static members in a separate class rather than an interface.

It was therefore decided not to permit static members of any description in interfaces defined in those languages.