I think that's purely because the leap from UC 1 to UC 2 was so drastic, that it's less noticeable between 2 and 3.
And for all the 'wow' factor we thought that U2 had over U3, I still think U3 has a vastly superior script when it comes to characters and their development. The way they cross examine the main protagonist was awesome... It had me realising he isn't just unlucky, all these situations that are life and death like collapsing buildings or sinking ships are exactly what he actually wants, he gets off on cheating death and this game is about him being forced to see how it affects those around him, who he cares about.
Such a great idea, I haven't seen anything close to that done from a character standpoint since GTA IV (and that was pretty heavy handed and overt, no subtlety to it, which can come off as just forced angst).
Really sterling script. My GF who is as much of a non gamer as you could ever see has become hooked on watching me play it simply for the story and characters. Great stuff.