Game Freak, early in development, made Eevee, and thought,
"What if we made Eevee evolve into a whole bunch of different Pokemon? Each Pokemon would have its own personality, and would really give a definition to 'Evolution'!"
Basically, a gimmick.
So they decided to make an Eeveeloution for each Specially-based type at the time (Fire, Water, Electric, Grass, Psychic, [the soon-to-be] Dark, Ice, Dragon).
And since a Dragon-type Eeveeloution would look as queer as the birds, then I suppose the Eeveeloutions are done!