Why did C++11 deprecate the default value in std::vector's fill constructor?

In C++98 the prototype for std::vector's fill constructor has a default value for the initializer:

explicit vector (size_type n, const value_type& val = value_type(),
                 const allocator_type& alloc = allocator_type());

In C++11 it becomes two prototypes:

explicit vector (size_type n);
         vector (size_type n, const value_type& val,
                 const allocator_type& alloc = allocator_type());

(In C++14 the fill constructor changed again, but it's not the point of this question.)

My question is, why did C++11 deprecated the default initializer value value_type()?

BTW: I tried with this code below in clang++ -std=c++11 and it issued an error, which means the value type still need to have a default constructor like S() {}.

#include <vector>

struct S {
    int k;
    S(int k) : k(k) {}
};

int main() {
    std::vector<S> s(5); // error: no matching constructor
}