Hades isn't considered one of the 12 Olympians. Being the god of the Underworld, he is not really a part of the upper realms, thus kind of ostracized by the other gods. He does not have a throne at Olympus and is only allowed to visit at the Winter Solstice. read more
In the starting, they have only cabins of Olympians, and considering Hades wasn't an Olympian, they didn't have a Hades Cabin. But after Percy's wish in the last Olympian, cabins of all Gods were made, even Hades. read more
Although Bianca di Angelo is a daughter of Hades, she never stayed in this cabin due to the fact that she joined the Hunters of Artemis, and died in battle during The Titan's Curse, and since the cabin was only built after the events of The Last Olympian. read more
In addition, after reforms made by Percy Jackson in The Last Olympian, cabins for Hades, Hecate, Nemesis, Iris, Hebe, Nike, Tyche, Hypnos, and many other minor gods were constructed. There are now a total of 20 cabins. read more