Harper Lee insinuates through the evidence shared in the trial that Mayella has been taken advantage of by her father. Young people who have been treated that way by adults tend to really struggle with trust, a required tenant of friendship.
Furthermore, Mayella's home life is not attractive. In order to spend time together as friends, children often attend each other's houses. Mayella's few rows of flowers demonstrate that she wants something better than she has, and therefore would likely be embarrassed to bring a friend home to be with her.
Her father's drunken nature makes her responsible for the other children most of the time. She doesn't have the time to have friends, she has always performed the duties of a mother ever since she was old enough to have friends.
Lastly, Mayella seems to lack certain social abilities that she should have at her age. She doesn't understand that when Atticus addresses her as "ma'am", he is doing so to show respect.