Why static variable needs to be explicitly defined?

In the class:

class foo
{
public:
    static int bar; //declaration of static data member
};

int foo::bar = 0; //definition of data member

We have to explicitly define the static variable, otherwise it will result in a

undefined reference to `foo::bar'

My question is:

Why do we have to give an explicit definition of a static variable?

_{Please note that this is NOT a duplicate of previously asked undefined reference to static variable questions. This question intends to ask the reason behind explicit definition of a static variable.}