We will get a book on this one day, but my speculation having had some dealings with egos of this sort, Kennedy definitely cornered out some personal stakes in star wars, she has a long and distinguished career, and she was putting the "Force is Female" stuff upfront and out front. But, with the rarest of exceptions (like mid 90's miramax), not one single person has ever bought a single movie ticket based on who the producer of a movie was. And I amend that number to zero based on who a production executive was.
JJ Abrams came into Star Wars with quite a shine, and to be clear, the number of tickets a director sells is relatively negligible, Star Wars sells Star Wars, not JJ Abrams. But JJ got much deserved credit and praise for The Force Awakens.
My speculation, based on his total divorce and absence from both the production company and Episode 8, is ego's clashed and she didn't like him getting the attention she got. Her hiring of RJ and pivoting dramatically from what was established let her put her fingerprints on it. Star Wars to that point, was a fool proof property, so she probably gambled there were no changes that could poison the well. She found out the hard way she was wrong.
Kennedy didn't know he role, her's wasn't the place for credit. It wasn't even her property, it was from George Lucas.
In Wag the Dog, Dustin Hoffman basically plays a Bob Evans character, and he remarks multiple times that no one knows what a producer does or words to that effect. Its the only job on a film set where you have no discernible skill or talent. If you have a checkbook, your'e a producer. If you get anointed, you're a producer.
BUT a good producer, is absolutely everything pulling everything together, it is the most important job at the same time, but from a credit perspective, its a bit nebulous. Its a major ego stroke but you have to be at peace without it being a scenario where you're getting public credit for it.