Answer:
It is due to more meristems present there.
Explanation:
Meristem tissue can be described as the undifferentiated cells of the plant which have the capacity for cell division and convert into different parts of the plant.
The growth is the most at the apical meristems. At the shoot tips, there will be more meristems as compared to the other parts of the plant. This is the reason that the top part of the stem will grow the most.