The King of Stormwind wears the crown on a troubled brow. He inherited the mantle as a child, not through a peaceful succession but through bloody violence and the destruction of his home. He wore it in exile and only came home with the death of the man who saved him and carried him away from the sight of his entire world burned to the ground. His entire life has been shaped by violent loss, by tragedy and death -- his mother dead before he even knew her, his father murdered and butchered in front of him, his replacement fathers cut down, his wife taken from him in a moment's passing by an errant rock thrown from a mob.
His early rule was most notable by his lack of desire to actually do much rulership, busying himself by riding the land in search of his father's killer or drifting though a haze of loss after his wife's death, a haze seized upon and manipulated by someone who was supposed to be a close advisor. The circumstances of his disappearance from the throne and his return have been discussed in detail. For now, all we need to do is accept that they did little to encourage him to view the throne as anything but a responsibility to be maintained in the face of constant peril.
Following the Northrend campaign and its heavy cost both to King Varian and the kingdom as a whole (Bolvar's death, as well as the many deaths at the Wrathgate; the invasion of Undercity and the destruction of Putress; Horde troops ambushing Alliance forces engaged with the Scourge; the astonishing cost in lives and resources), it would have been difficult for either the King or the kingdom to quickly recover. The eruption of Deathwing and the Cataclysm he caused did not allow the luxury of time. Reeling from one blow, they suffered another and another.