A very good question. Here are some possibilities:
1. Juliet is very inexperienced. She only leaves her house to go to church. She does not know enough to get her from Verona to Mantua. She wouldn't know what to do. Of course, if she was sent in the care of a friar or nun this wouldn't be a problem.
2. After Tybalt's death, Mrs. Capulet has a real hate on for Romeo. She says, "I'll send to one in Mantua, where that same banished runagate doth live, shall give him such an unaccustomed dram that he shall soon keep Tybalt company." If Mrs. C had any idea that Romeo was her son-in-law, she'd want to kill him that much faster, one would think.
It's still not totally convincing. One would think that there might have been other ways to get Juliet to Mantua safely. What if she pretended to retire to a convent, then went to Mantua when nobody was looking?
Juliet's parents' lack of knowledge about her secret marriage to Romeo, their rigid opposition to the union, and their forced engagement for her to marry Paris are the main factors preventing her from joining Romeo in Mantua. Additionally, the societal expectations and restrictions on young women during that time period also contribute to her inability to easily leave Verona and be with Romeo.
Mantua. It's not far from Verona.
Mantua
Go to Mantua together.
Romeo was banished from Verona and sent to live in Mantua.
He went to Mantua.
He isn't banished anywhere he is banished from his town that he is currently living in. Meaning he can go anywhere else. *He is banished to Mantua
Balthazar.
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.
Live happily together in Mantua.
Balthasar.
After Juliet wakes up, Romeo is supposed to take her to Mantua
Juliet lived in Verona aswell as romeo before he got banished to mantua.