Lots of things are omitted for the Palm Sunday Gospel reading.
From the Roman Missal:
“21. The narrative of the Lord’s Passion is read without candles and without incense, with no greeting or signing of the book. …
22. After the narrative of the Passion, a brief homily should take place, if appropriate. A period of silence may also be observed.”
From the Ceremonial of Bishops:
“273. After the verse on the death of the Lord, all kneel and a brief pause is observed. At the end of the words This is the Gospel of the Lord are said, but the book is not kissed.”
We now say “The Gospel of the Lord”.
The “signing of the book” is described in General Instruction of the Roman Missal n. 134:
“Then he says, A reading from the holy Gospel, making the Sign of the Cross with his thumb on the book and on his forehead, mouth and breast, which everyone else does as well.”
So why omit these things. It highlights that this is an unusual Gospel reading, the Passion. The option to omit the homily is probably because the reading is longer than normal.
It could be argued that omitting the other things saves some time. But then the kneeling takes extra time. So I think it is about doing things in a different way, reminding us that this is a special Gospel reading.