The main 2 reasons are probably:

Cost:
The case has to be made stronger, (To support the hinges) as well as re-designed both electrically and physically.

Sales:
Why would you buy a T4 if you could get most of it (Including a tilt screen) for half the price?

As to the touch screen, if you read on most dedicated Yamaha forums (The ones that Yamaha takes note of) you will see it is still at about 50 – 50 for this option.

Bill