Because he thinks Juliet is dead. Remember that Friar Laurence's plan is to give Juliet the sleeping potion, and then let her family discover her body. When she is placed in the family tomb, Friar Laurence and Romeo will be there to see her awake - and then Romeo and Juliet can spend their lives together, far away.
This works excellently: except that Friar Laurence's letter to Romeo, explaining the plan and explaining - crucially - that Juliet is not dead, does not get delivered. Friar John is imprisoned in a house because of the plague, and cannot get the letter to Romeo. He returns it to Friar Laurence.
Romeo hears from Balthasar - who has heard it from someone else in Verona - the news of Juliet's death. He resolves to kill himself next to her body, to lie with her in death. Consumed by grief, he buys poison and drinks it next to her body. And then - tragically - she wakes up to discover his dead body: and kills herself.
Because he believes that Juliet is dead.
Remember a messenger came from Verona with the news that Juliet is dead, which sends Romeo half mad with grief. He does not know that she is not actually dead, but only under the effects of a drug that Friar Lawrence has given her to help her to fake her death and thus to avoid having to marry Paris.
Later she will wake up, but sadly only after Romeo has taken the poison and died.
So Romeo goes to her tomb prepared to commit suicide, having already brought the drug he is going to use from an apothecary.
Romeo drank the poison because he thought Juliet was dead . Previously , Juliet drank this potion that makes her seem dead for some hours . After the Capulet found out that Juliet was " dead" they placed her in the Capulet's morgue . Then , Romeo comes visit the morgue and sees Juliet there .
because he thinks that juliet is dead due to the potion that frair laurence had given to her to fake her death. Romeo did not get the letter that juliets death was a play