Because it avoids this:
class A {
public:
A(int) {}
};
class B0: virtual public A {
public:
B0(): A(0) {}
};
class B1: virtual public A {
public:
B1(): A(1) {}
};
class C: public B0, public B1 {
public:
C() {} // How is A constructed? A(0) from B0 or A(1) from B1?
};