Does anyone know the rational for why you can't explicitly specialize a template within a class/struct?

Question

Can anyone tell me the rationale for not allowing explicit specialization in non-namespace scope? Why must I do:

template <typename ...Ts>
struct types;

template <typename T, typename ...Ts>
struct types < T, Ts... > : public types<Ts...>
{
    template <typename F, int i = 0>
    struct find : public types<Ts...>::template find<F, i>
    {
    };

    template <int i>
    struct find < T, i > : public std::true_type
    {
        typedef T type;
    };
};

template <>
struct types < >
{
    template <typename F, int i = 0>
    struct find : public std::false_type
    {};
};

demo

and not:

template <typename ...Ts>
struct types;

template <typename T, typename ...Ts>
struct types < T, Ts... > : public types<Ts...>
{
    template <typename F>
    struct find : public types<Ts...>::template find<F>
    {
    };

    template <>
    struct find < T > : public std::true_type
    {
        typedef T type;
    };
};

template <>
struct types < >
{
    template <typename F>
    struct find : public std::false_type
    {};
};

demo