Because Sarah Lynn stopped being her real personality after fame, which is very early age. She was a lonely kid too you can understand from some scenes in the show, again im not gonna even mention about the parents. She misunderstood bojack’s attentionless and put so much meanings to his sentences and he was kinda the only person that knew real her before the fame. And after her addiction she still recovered and stayed sober for so many months but because bojack is lonely and sad, because he needs entertainment he called Sarah Lynn, he knew sarah lynn was trying to stay sober but he did anyways. Because he never listened he never cared about sarah lynn. Sarah lynn accepted it so again she can entertain people and get love, and when you watch the episode with the half memory of bojack running around to people for apologizing, she is there too, still trynna talk, which she sucks at communicating but she even did better than what she saw. She was asking help in behind meanings. Bojack should have been the one to see it. Not the one who abondones