Question

why it is better to transplant seedlings in the e evening and not in the morning?

Answer 1

In the evening, the stomates of the plants will close, and this will slow the loss of water through the leaves.  Since the roots may be stressed by the transplanting, doing this in the evening will help prevent too much water loss and wilting in the newly transplanted seedlings. In the morning, the stomates will be open for an entire day, losing water vapor all the while.  So this is not the best time for transplanting.

Answer 2

it is better to plant seedlings in the evening. This will give the tender plants a chance to settle in a bit before the relative harshness of daytime comes.

Evening planting of starts is especially important if the daytime temperatures are extreme, or the conditions into which they are planted is significantly different than where the starts were raised.