Call of Duty: Black Ops II was the most demanded game on Xbox's feedback site where people could vote on which games they would like to see become backwards compatible. From the list's inception and up until this point, Black Ops II was the most requested game, with over 211,000 votes from fans.
On Twitter, Xbox and public figures that work for Xbox have been bombarded with Black Ops II tweets, which have gained traction over time. It became so annoying that at one point Xbox Live's Major Nelson pleaded for people to stop asking him about it. Xbox Head Phil Spencer has said he really wanted to see it too come, and loved the passion the fans had for the game.
Anywho, hope for it to arrive had died down for a while, with some thinking it would never come. But with the additions of fan-favorites such as Black Ops and World at War coming along, it at least showed that Activision has supported backwards compatibility (the publisher is the one who gives permission to which titles are eligible to be backwards compatible)
On April 7, 2017, a bombshell of an announcement came with Microsoft anoucning that Black Ops II would be available on Xbox One, along with its DLC. Population has skyrocketed as of time of writing this (and looks to be going even higher) at 70,000 players concurrently online.
In the meantime, this adds to the list of Call of Duty titles available on Xbox One, with Call of Duty Classic, Call of Duty 4: Modern Warfare (this title will probably be the least likely to return dye to it's recently released remaster), Call of Duty: Modern Warfare 2 and Call of Duty: Modern Warfare 3.