Because she turned 30.

Well, the porn star parents thing is true. There's an English actor by the name of Tyger Drew-Honey whose parents are also porn stars, shares a similar upbringing.

The thing is, it's not her upbringing that is causing issues, nor is it her parental background. I mean, this is hollywood, where they will hire anybody from drug addicts to convicted paedophiles (Victor Salva, who directed Jeepers Creepers, served a jail sentence in 1988 for filming himself whilst molesting a 12 year old boy, yet he is still directing) as long as they can do the job required.
What derailed Birch's career is the fact that her dad is a complete control freak when it comes to his daughter, so much so that she was even fired from a Broadway production of Dracula, because her dad started threatening one of the actors. It involved the actor having to massage Thora's back (As part of the scene) and her dad just went ballistic on this guy.
The director had to place the protection of the cast ahead of Birch's acting (as a negative atmosphere will affect the entire play) and so she was fired a few days into rehearsal.
Directors will tolerate a lot of things from actors, but not when it affects the rest of the crew. And not when they are so difficult that the people in charge will seek out an alternative. It's the reason Sean Young cannot get arrested, Stuart Townsend is begging for TV roles, and Katherine Heigl has retreated back to TV (her and her mom are notoriously difficult) their actions far outweighed their performances.