Ubuntu has been one of the last distributions using Upstart, which they maintain at Canonical, rather than switching to systemd like many others have done. However, more and more software from GNOME and other vendors is dependent upon systemd. The nail in the coffin though was Debian's decision to use systemd. With Debian switching to systemd and Ubuntu being derived from Debian, they have decided to go ahead and adopt systemd.
Mark Shuttleworth wrote, "the decision is for systemd, and given that Ubuntu is quite centrally a member of the Debian family, that’s a decision we support. I will ask members of the Ubuntu community to help to implement this decision efficiently, bringing systemd into both Debian and Ubuntu safely and expeditiously. It will no doubt take time to achieve the stability and coverage that we enjoy today and in 14.04 LTS with Upstart, but I will ask the Ubuntu tech board (many of whom do not work for Canonical) to review the position and map out appropriate transition plans. We’ll certainly complete work to make the new logind work without systemd as pid 1. I expect they will want to bring systemd into Ubuntu as an option for developers as soon as it is reliably available in Debian, and as our default as soon as it offers a credible quality of service to match the existing init."
Mark wrote the details of their plans to switch to systemd in a blog post entitled Losing graciously. The migration won't be done in time for Ubuntu 14.04 LTS but at least Canonical has finally given in and will be moving to systemd.