Why must virtual base classes be constructed by the most derived class?

The following code won't compile:

class A {
public:
    A(int) {}
};

class B: virtual public A {
public:
    B(): A(0) {}
};

// most derived class
class C: public B {
public:
    C() {} // wrong!!!
};

If I call A's constructor in C's constructor initialization list, that is:

// most derived class
class C: public B {
public:
    C(): A(0) {} // OK!!!
};

it does work.

Apparently, the reason is because virtual base classes must always be constructed by the most derived classes.

I don't understand the reason behind this limitation.