People always point to the fact that Eeveelutions have always been special types previously. However, their types do have another quality in common, one that I think is more valid considering that Physical and Special types are now meaningless.
They are all resistant to themselves.
Water, Electric, Fire, Psychic, Dark, Grass and Ice are all "not very effective" against themselves. Only two types with this qualkity have not been made into Eeveelutions: Poison and Steel. All other types deal regular damage to their own type, or are strong against it in the case of Dragon and Ghost. In fact, this is how I remember which types are resistant to themselves.
Of course, this could easily be a pure coincidence, but then, so could the Special types theory. And I don't know about you, but I find Poison and Steel Eeveelutions a lot easier to imagine than Dragon.
I could be wrong. For all I know, they could be planning on making 16 total Eeveelutions, one for each type, except Normal. (I'm probably in the minority when I say I think that'd be really awesome.) But anyways, I personally think this theory makes more sense than the Special one, especially since Physical/Special types are now nonexistant.