It seems that more and more are abandoning the expense of selling games on Steam. Blizzard runs battle.net which hosts several of their games already. Now Activision is looking to cut distribution costs as well.
We will know more when the publisher reveals the details of the Call of Duty: Black Ops 2 on May 17, 2018. Until then the hype machine is running on overdrive.
Steam is comparatively expensive and more and more publishers are jumping ship. So far Steam has not taken any steps to win back publishers such as reducing costs etc.
EA was the first to open up Origin to sell their games. They have also listed a few additional third party games for sale however it seems that EA has not made a commitment to competing with Steam so far.
Perfect World long ago established their own store. They operate entirely on the free to play model which has been surprisingly successful. Blacklight: Retribution was very popular but it has since been taken over by a new publisher.
Ubisoft stepped up with their uPlay store and they gave away several classic games at no cost to gamers. Ubisoft has been publishing games for many years and the cost of Steam has become too much to bear.