Because people don't take notice of the plain colours, shades and shapes on Darth Vader's sprite (or don't mind them) and are sold on the pose.
Or because it shares more aesthetic elements common to D/P sprites than any of the other entries?
It's fine if you think Wyverii's sprite is the most aesthetically pleasing in general. After all, all art is (reasonably) subjective. However, we are making a pokemon, and I am going to vote for the sprite that looks most like it came from a pokemon game.
And yes, there is no mandate that says our sprite should look like it came from a pokemon game. However, since it's going to be played with other pokemon, have concept art that is intended to look like a pokemon, have a name in the pokemon style, have pokedex entries like a regular pokemon, etc. etc..... I'm going to vote for the entry that most looks like a pokemon sprite.
You could argue that the Mona Lisa is more aesthetically pleasing than Guernica. I don't mind, that would be your opinion; there's no way to prove one is better than the other. But, if the challenge were to create a painting in the cubist style, then Guernica would win hands down, and I'd consider anyone to think otherwise a fool.
And if you're going to underestimate the value of simplicity in shape, color, and design, then maybe you should brush up on your art basics...
(
EDIT: Although, Wyverii's incredible backsprite makes the decision even more difficult...)