A lot of that comes from the story and script and a lot of it is my own personal aesthetic. Part of it is just a fad — generally speaking stuff that is blocky or angular like the original trilogy Star Wars stuff is looked at as being kind of "old school" and can therefore be dismissed by directors as such when they want to appear cutting edge.
The script almost always calls for "something we've never seen before" and many people in control think that it [the blocky angular look] is played out or something. I couldn't disagree more, but then I'm not a director — I just come up with the stuff! The more curvilinear and subtle stuff is much harder to do as a practical model or in CG and therefore tends to look "newer." Sometimes it's as simple as that.