Why can't an inherited protected constructor be made public?

Unlike other members, the accessibility of the using-declaration that introduced the inherited constructor is ignored.

[namespace.udecl]/19,

(emphasis mine)

A synonym created by a using-declaration has the usual accessibility for a member-declaration. A using-declarator that names a constructor does not create a synonym; instead, the additional constructors are accessible if they would be accessible when used to construct an object of the corresponding base class, and the accessibility of the using-declaration is ignored.

Actually, the inherited constructor can be made public, but not just the way you wrote it. You can define your B class as follows:

class B : public A {
public:
    B() {}

    B(int x) : A(x) {}  // instead of using A::A(int)
    using A::f;
};

(see it on GodBolt)

Perhaps the standard committee thought that saying using A::A would be a bit ambiguous, since a constructor of the base class is not exactly the same thing as a constructor of the subclass.