Refinery29 reports that Aniston was already committed to another project when she was cast as Rachel Green in Friends. She had already filmed six episodes of an unaired CBS sitcom called Muddling Through before starting on Friends—and if that show had been picked up, she would have had to leave Friends halfway through the first season.
But the studio wasn’t going to lose Ross's lobster so easily. Saul Austerlitz covered the conflict in his book, Generation Friends:
“CBS, after some dithering, ultimately chose to put [Muddling Through] on its summer schedule, in the relative dead zone of Saturday nights. Hearing the news, [Warren] Littlefield turned to [Preston] Beckman, NBC’s scheduling guru, with a two-word order: ‘Kill it.’"
NBC decided to release TV films adapted from Danielle Steele novels at the same time Muddling Through was airing. As Austerlitz writes, "They were practically guaranteed to attract a substantial, and substantially female, audience. ... no show about an ex-con motel manager and her daffy family was likely to provide stiff competition for Steele’s glamorous romances."
The films crushed Muddling Through, and Aniston stayed put on Friends. The sitcom launched her career, and is still adored by viewers to this day—it's even spawned a number of weird fan theories. Thank goodness for the cutthroat NBC executives who helped us keep the one and only Rachel.