Unlike the back triggers being analogue on both - the pressure sensitivity on the face buttons is probably not used because the Xbox 360 controller has digital face buttons.
And basically, when games are on multiple platforms there's usually the lowest common denominator issue (though the decision to require gamepads on PC helps that here). The WiiU gamepad and pro-controller has digital triggers - so that would create a massive design issue if you were targeting that console as well. Which is insane when you consider the Gamecube basically perfected analogue trigger buttons (though stuffed things up with the awful D-pad).
Can't wait to see how the Xbox One controller feels - it looks like they fixed the D-pad, which was my main complaint with the 360.
Seriously, all they needed to do was make the Dreamcast controller smaller and add bumpers to it - is that too much to ask?
/controller-rage.