Why couldn't friar Laurence's message reach Romeo in mantua?

He wasn't given the letter because Friar John was supposed to deliver it. He visited someone on his way there that was effected by the Plague. Therefore, he was quarantined. Romeo was told by Balthasar (Romeo's Man) That Juliet was dead. Romeo rushed over before he had a chance to be delivered the Friars letter.

Hope this helps. :]