Почему шаблонный параметр-шаблон не принимает другие параметры шаблона

template <typename T,
    template <typename _Ty, class _Aloc = allocator<_Ty> > class CONT = vector>
class SomeContainer

template <typename T,
    template <typename _Ty = T, class _Aloc = allocator<T> > class CONT = vector>
class SomeContainer

template <typename T,
    template <T, allocator<T> > class CONT = vector>
class SomeContainer

template <typename T,  template <typename, typename> class CONT = vector>
class SomeContainer

template <typename T,
    template <typename _Ty = T, class _Aloc = allocator<_Ty>> class CONT = vector>
class SomeContainer

CONT<T> elems;

CONT elems;

#include <iostream>
#include <vector>
 
template <typename T,
    template <typename _Ty, class _Aloc = std::allocator<_Ty> > class CONT = std::vector>
        class SomeContainer1 {};
 
 
template <typename T,
    template <typename _Ty = T, class _Aloc = std::allocator<T> > class CONT = std::vector>
        class SomeContainer2 {};
 
 
int main()
{
    std::cout << "Hello, world!\n";
    
    SomeContainer1<int, std::vector> vec1;
    SomeContainer2<int, std::vector> vec2;
}

#include <vector>
 
template<typename T,
         template<typename TT, typename A = std::allocator<TT>> class CONT = std::vector>
class SomeContainer
{
    CONT<T> c;
};
 
SomeContainer<int> sc; // OK

#include <vector>
 
template<typename T,
         template<typename, typename> class CONT = std::vector>
class SomeContainer
{
    CONT<T> c;
};
 
SomeContainer<int> sc;
/*
 prog.cc:7:5: error: too few template arguments for template template parameter 'CONT'
    CONT<T> c;
    ^
 prog.cc:4:45: note: template is declared here
         template<typename, typename> class CONT = std::vector>
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~       ^
*/