IO went through a good 12-18 months as Absolution wrapped up/released where they weren't working on Hitman, but were working on a something else - either new IP, or new additions to their other franchsies like Kane & Lynch, Mini Ninjas (surprisingly successful), or Freedom Fighters (a cult classic). Square Enix Montreal (not to be confused with Eidos Montreal - different studios in the same building) was handed the Hitman license. After a bit, whatever SE Montreal was doing with a full-fledged Hitman wasn't working, and the studio was significantly downsized with a bunch of lay-offs.
Core Hitman development was shifted back to IO, a number of IO employees were laid off, and all their non-Hitman projects were shelved. At least one of them was, I believe, a 'big' game - some of the work on it may have transferred directly into Hitman, however... but who knows, depends what type of game it was - if it was Kane & Lynch, for instance, you could certainly retool engine & asset stuff for a Hitman game. SE Montreal still worked with Hitman despite their downsizing, and put out Hitman Sniper and Hitman GO for mobile.
Point is, anyway, Hitman's been a bit iffy and while IO has been doing a lot of work, they probably aren't as far along as Deus Ex. Think of Eidos Montreal as being like a smaller version of Bioware - for instance, the 'Mass Effect Team' worked on Dragon Age Inquisition helping out, but there was a core, fenced-off staff working on ME4 at the same time. Deus had lots of engine & universe work done well in advance of Thief releasing, and that will have allowed the Thief team to move swiftly into that new game and start creating assets quickly... whereas the IO situation was plainly a lot more complicated and difficult.
However, in spite of all this, a Hitman game might well end up being pushed to an earlier reveal because 2015 is the new film reboot release. The previous film did surprisingly well (it grossed 100 million off a 25 million budget), and they may want to at least announce a game to ride the coat tails of the film:
Between this and the fact that we know a Tomb Raider script is being shopped around Hollywood again and the Deus Ex film rights sold, it seems like Square's Western properties are going to have a big cross-media resurgence - ironic really considering among gamers (correct or not) the Japanese are the properties more renowned for story.