#1. To maintain a sense of uniformity
If you've noticed something about WWE programming today, it's that everything must be presented with a sense of uniformity. From the presentation of WWE superstars to the commentary to the promos, the programming itself is quite formulaic and they probably use that because in their eyes, it's a tried and tested method.
This uniformity is something that Vince McMahon seemingly takes to heart and it's known especially by the commentators that he is always in their ears during live shows and is constantly giving instructions.
Since they want to apply that uniformity to the entire product as a whole, it leaves no space for cutting off-script promos and improvisation. While it is somewhat understandable from WWE's perspective, it has often led to rather monotonous programming on many weeks. Hopefully, they do give leeway of some sort to superstars to go off-script time and again.
Also read: 5 reasons why Roman Reigns is planning to transition out of WWE in 2 years