Why did she stop acting ?

This Reddit post may provide some small insight:

https://www.reddit.com/r/movies/comments/1do60t4/comment/laaajym/

Bijou Philips. Putting aside her defending Danny Masterson, she was a Paris Hilton personality before Paris Hilton. Notoriously hard partying and relentlessly bad attitude on multiple sets and all round difficult to work with, especially on her first major breakthrough Bully. Larry Clarke and all the cast knew she had to be there but they all couldn’t stand her. She frequently bullied Daniel Franzese on set with fat shaming and homophobic taunting trying to goad him into coming out on set, and it took Michael Pitt (ironically), Brad Renfro (who was starting to have massive issues with substance abuse) and again ironically Nick Stahl (who was playing the titular bully in film) all repeatedly telling her to shut the fuck up.