Answer:
Because she felt guilty and ashamed.
Step-by-step explanation:
In Persepolis, Marji was depicted as a young girl who is approached by God in her dreams. These constants dreams made Marji believed that she's destined to be a prophet by the time she's 6 years old.
On an occasion, she told her friends and her teacher about her dream. She said that she wanted to be a prophet when she's grown up. But none of them believe her and taught that she's mentally unwell.
Because of this, Marji decided to lie to her parents about having the dream.