[boost] [fusion][containers]reason for not reusing mpl::vector to specify element types

Both fusion list and vector specify argument types with: template < BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT( FUSION_MAX_LIST_SIZE, typename T, void_) > struct container_whatever; where container_whatever may be either vector or list. What's the reason for not using mpl::vector instead of using yet another BOOST_PP_ENUM* call and requiring another *_MAX_*_SIZE macro?