In Standard, Kalitas, Traitor of Ghet proved himself to be a major competitor, making him a $20-25 card in mid-April 2016. He’s pretty good in Modern, too! Because of this, his price tag put him a bit out of reach for some Commander players for some time. When he left Standard, however, his price dropped closer to $12, allowing for more people, especially EDH players, to fit him into their collections.
There are so many good mono-Black Commanders already. But Kalitas not only can get you an army of Zombie tokens while shutting off any graveyard-based strategies. He can also make himself bigger, meaning he could potentially take out players on his own.
So, why hasn’t Kalitas, Traitor of Ghet become more of a player in EDH where it seems like he would be very powerful? Part of the problem is that he prevents any decks that work at all out of the graveyard to function, while still being able to use your own graveyard. Basically, he can draw hate very quickly. As a member of the 99, he’s still quite useful, but again he draws a lot of hate.
Kalitas, Traitor of Ghet is definitely one of the more powerful Legendary creatures from recent sets. The pieces are there for him to lead formidable decks, and perhaps a bit too formidable for your everyday Commander group to want to deal with on a regular basis.
EDHREC has a good sampling of decks with Kalitas, Traitor of Ghet as the Commander. You can check out their Kalitas, Traitor of Ghet page to get an idea of what sorts of EDH decks people build around him.