It wasn't just the crappy ending. LadyGT was bullied by rabid fans who hated NaruSaku and her art. Also, she felt really disappointed when the author himself talked BS about the pairings. You know, red herrings, Sakura being a terrible woman if she forgot about Sasuke, every contradiction... It just didn't feel right anymore. I mean, she spent years following this series, improving her Naruto's drawing style and defending her OTP, just to get this in the end.
She decided to move on and I can't blame her for that. Boruto sucks.