Because Arceus can only be obtained at Level 100, meaning it can't be EV trained regularly - you can only EV it using vitamins (eg Carbos). The EVs given by vitamins cap at Level 100.
EDIT: Sniped.
EDIT2: Okay. Guys. Seriously. How the hell do you actually deal with Ho-Oh? I mean, it's so bulky that it's nearly impossible to revenge-kill, its STABs are so strong and it keeps demolishing me because I'm not prepared to run ScarfTar. Palkia can't deal with it, Latias can't deal with it, Giratina-O can't deal with it - how can I cover Ho-Oh at the team-building stage?
EDIT: Sniped.
EDIT2: Okay. Guys. Seriously. How the hell do you actually deal with Ho-Oh? I mean, it's so bulky that it's nearly impossible to revenge-kill, its STABs are so strong and it keeps demolishing me because I'm not prepared to run ScarfTar. Palkia can't deal with it, Latias can't deal with it, Giratina-O can't deal with it - how can I cover Ho-Oh at the team-building stage?