Fire cracker, it's because with ev training, every pokemon will have the best stats possible for it's "job." for example, a pokemon designed for sweeping, such as Excadrill values Speed and attack over HP or either of the defenses. Since you can only have enough evs to max two skills, Attack and Speed are the best way to go. However, a wall, such as Blissey, is more useful if it has a lot of HP and Defence or SpDef. Therefore, people will maximize Blissey's HP and SpDefence so it can be most useful.