well, he was made mentally inept by the "elder gods" in the war he waged with them, so he's both horribly brain damaged, and very tired.
a creator deity "who made the gods and thereafter rested."
Outside the ordered universe is that amorphous blight of nethermost confusion which blasphemes and bubbles at the center of all infinity—the boundless daemon sultan Azathoth, whose name no lips dare speak aloud, and who gnaws hungrily in inconceivable, unlighted chambers beyond time and space amidst the muffled, maddening beating of vile drums and the thin monotonous whine of accursed flutes.
so in other words, the weaker gods are keeping him asleep, otherwise..