there are 2 reasons that I know of for sure. They were on either wikipedia or another one of her websites.. she refused to re sign an extra 2 year contract with the show and she always started fights with Alyssa Milano so they fired her..
first of all Shannen Doherty is not at all mean or anything like that. She and Alyssa where having problems and she just got over all the fighting and said this is getting way too problematic, so just let me go, and let it be over with. and they did, so Prue died.
from what I know it was Alyssa that refused to work with Shannon anymore... I know Shannon's not the kindess person but she's still a great actress and to refer to her as a 'bitch' is just wrong if it wasn't for her the show wouldn't have been as successful as it was in the first 3 seasons she helped start it where Rose helped finish it... I know everyone has there own pov when it comes to this, this is just mine :)
because she had a contract so she decide to leave to find more in the world her co-star Holly Marie Combs who plays Piper want to leave to but she had a contract for eight years they were like sisters in real life.
Shannon is extremely high maintenance and very bitchy. I love the show and everyone in it. I still watch it daily, but she was fired for it so obviously she wasn't the easiest to work with. After which she started her own show, can't remember the name but it wasn't on long at all. Not even two weeks I think. And she ain't been seen in anything hardly since except adds and infomercials that I know of. That's how hard she is to work with. I don't think bitch is a harsh term at all. She'll even tell you herself she's a bitch lol. And that along with her cockiness and high maintanence attitude pretty much killed her career. The only thing she's had successful since then is her reality show and that is because it's ALL about HER. Like I said I love charmed and her character and I wish they coulda brought her back for a reunion type thing or movie but I doubt it'll ever happen because of how she is. The only person she liked and got along with was Hollie (Piper), and that's because they were friends before the show. Hollie actually got the part of Piper because of Shannon.
Shannon Doherty didn't actually leave the show. There was a problem with Shannon and Alyssa Milano. Basically, Shannon told the producers that it was either her or Alyssa and when Shannon left (I think she was filming something) she got a call that they were going to kill off Prue. According to Holly Marie Combs (Piper) she didn't like the way they handled things. Another sense of tension between Alyssa and Shannon was that Shannon started dating Julian McMahon (Cole) while his and Alyssa's characters were together.