Because life isn't fair
The letter that was supposed to be delivered to Romeo informed him of Juliet's fake death plan. However, due to unforeseen circumstances, the letter did not reach Romeo which led to tragic consequences.
It explains why the friar's message did not reach Romeo
Friar Lawrence wants a crow as a messenger to send a letter to Romeo informing him about Juliet's fake death plan. He believes that this bird will reach Romeo quickly and help prevent tragedy.
In act 5 of Romeo and Juliet, Friar Lawrence did not consider that his plan to fake Juliet's death with a potion might not reach Romeo in time or that miscommunications could lead to tragedy. Additionally, he did not anticipate the unforeseen events that ultimately led to the deaths of Romeo and Juliet.
Friar Laurence made a plan with Juliet to stop her from marrying Paris. He tells her to drink a potion which makes her seem dead. He then tries to send a message to Romeo telling him that he has to come to fetch Juliet and they can be together. The message doesn't reach Romeo and he then thinks Juliet is dead. Then they kill themselves. You should really watch one of the films or the play.
Mantua: City in Italy where Romeo flees after saying his last goodbye to Juliet. He hides here, and waits to hear from the friar. It is here that he learns that Juliet is supposedly dead. Then, on his way to go and see her, he buys poison from an apothecary on the street.
He penned a letter and gave it to Brother John (?) and asked him to give it to Romeo. Unfortunately, Romeo didn't receive the letter and the rest is history.
Friar Lawrence's plan failed because Balthasar told Romeo of Juliet's death before the Friar's message reached him. Had Romeo received the Friar's message before he left Mantua he would not have killed himself.
We all know that Juliet had drugged herself to put her in a temperary state of death so Romeo could come and rescue her. Though, Romeo only got the message that she was dead, not the fact that it had been faked. So, he brought poison to her crypt, laid down beside her, and drank the poison, killing him. Soon, Juliet had woken up, seen the goblet of poison in Romeo's hand. Knowing he was dead, she took a dagger and stabbed herself.
There are several instances of coincidence in Shakespeare's "Romeo and Juliet," such as Romeo and Juliet's chance meeting at the Capulet's party, Friar Laurence's message failing to reach Romeo in time, and the timing of Juliet's awakening from her potion-induced slumber coinciding with Romeo's arrival. These coincidences help drive the plot forward and create dramatic tension in the story.
Only Friar Laurence and the audience knew that Romeo and Juliet were going to die. Friar Laurence was aware of their plan because he helped them with it, and the audience knew because it was revealed in the prologue of the play.
The nurse in the play Romeo and Juliet has a motherly relationship with Juliet despite being the family's servant. According to the nurse, Juliet had yet to reach her 14th birthday while she secretly made wedding plans to marry Romeo.