Because there were complaints when Wrath and Cata** did not have any world bosses. Even though raids like Vault of Archavon and Baradin Hold were designed to be replacements for world bosses. Although having it tied to a pvp event was not ideal.
** Cata did have “world bosses” but they were more like ultra rare spawns that were designed to be done by a group of 5-10 players. Although you could bring more. There was one in each leveling zone. For example Garr (yes that Garr from Molten Core) was the one for Mount Hyjal.
https://warcraft.wiki.gg/wiki/Garr_(world_boss)