the studio is used to this engine and probably dont have the time nor the ressources to reform their whole team to id tech 5 just because its an engine made by a zenimax media owned studio
That's not the way I'd have said it, but yeah that's definitely one reason (among many others).

I don't want to start a flame war or anything so just consider the following situation: you're playing a pure shooter (something such as, let's say, Bulletstorm). You enter a super high tech laboratory filled with tubes and jars. Enemies are coming in, bullets start flying everywhere, with a lot of smoke and particles emitted in the air... And when you finally kill your last opponent, everything returns to calm again. Now look around you: wouldn't you like the glassy stuff to be broken down with pieces of glass lying everywhere on the floor? Well, that's exactly what you'd expect to see in an immersive sim such as Dishonored, and that's the kind of stuff which is easier to achieve with Unreal (and its built-in tools) than with a lot of engines out there. Because we'd like the whole world to be dynamic and to react to the player's (and AI's!) actions the same way on consoles and on PC.
And don't get me wrong: that does not mean Dishonored is a heavy shooter.
