-The Original One-

(c) Tossu-sama 2012
All rights reserved.

"Born from emptiness, I am the Original One
The supreme being, the ultimate Alpha
From my own body I created matter and antimatter
From my own mind I forged knowledge, emotion and will
By my command the universe flows
By my judgment the evil shall be punished
For I am the Original One, the ultimate Alpha"

***

Because Arceus is the god of Pokémon universe.

About the design:
The so-called outfit isn't actually an outfit at all. Arceus is the Original One, a godly being in the Pokémon universe so I think it's okay for it to have very peculiar body which, in my view, has taken a form that resembles a futuristic outfit of some sort.
The helmet-like headgear isn't actually a helmet, it's growth in Arceus' head, like the gloves, boots etc are just natural parts of its body.
I was very keen on keeping the circle part of Arceus' body in the gijinka design so I moved it behind its back, making it to be attached to its waist.
As you can see, Arceus doesn't have very much of a face... Well, pretty much the same explanation as for Regigigas. Arceus is a divine deity, a mysterious being who for some bizarre reason decided to create the world so a face with distinctive features would dictate its personality. I don't want to do that. Everyone can think themselves what kinda personality Arceus has.
The "jewels" in the yellow circle and face are in my version the indication when Arceus is using its powers. They start to glow whenever the Alpha decides to engage in some godly badassery.
The yellow and green parts will change color according to Arceus' type which (as we all know) changes according to the plate it's holding. Multitype ftw.

The glowing circle in the background is traced from this one: [link]
The starfield is completely self-made! Hail my digital skills!

Can you tell I like making dramatic pictures?

0.1 Pigma Micron
GIMP 2

Art & design (c) ~Tossu-sama
Arceus & Pokémon (c) Nintendo/Game Freak