Why non-static data members of class cannot be given default values using '( )'?

I'm just going through the basic OOP concepts in C++ and came across the following:

class A{
    public:
    int i(20); //line 1
};
int main()
{
  int j(20);
  cout<<j<<endl;
  A obj;
  cout<<obj.i<<endl;
}

I get the following error at line1 when compiling (tried in both gcc and MSVC++),

expected identifier before numeric constant

I know how to assign default value for a non-static member (which can be done from C++11 on wards), but I couldn't get the reason why only this kind of default value initialization is not possible as doing the same initialization (for normal variables) anywhere else is valid.

What could be the reason for such restriction?