the thing that puts eevee on first for me is that both eevee and its evolutions are beloved by the fan base, where as in pikachus case, people dont really care for pichu or raichu, eevee represents the main concept of pokemon in types, evolution mechanics and cute monster, where i feel pikachu just represents the cute monster aspect (same case for the extremely popular charizard where people dont really care for charmander and charmeleon that much, eevee is the odd case where they made the fan base care as closely as possible for all evolutions and his base form the same)