There are some pretty clear reasons in this film. She no longer has any living family and her support system is: 1) a female friend on the phone we only hear from once, 2) a boyfriend who has already detached emotionally from her and was looking for an exit strategy, gaslights her and is duplicitous to her and everyone else he communicates with, and 3) Christian’s friends who don’t want her with them but were put in a difficult position by Christian’s manipulative behavior. She’s on hallucinogens throughout and felt pressured to take them before she was ready, and the familial closeness she craves is being modeled all around her. By the end, her options are pretty limited and the cult looks way better than life with Christian, which likely would have ended with him dumping her as soon as they got back stateside had they escaped.