According to Reuters, they were leaving before Musk bought the company because their promoted tweets were being shown alongside those advocating pedophilia.
Later, many didn't want to advertise out of fear that their promoted tweets would be shown alongside those showing photos of dead Palestinian children.
Additional points:
Most of the owners were eager to sell to Musk because the company was overvalued (it lost money almost every year it operated, and during the only time it earned big it lost big the following year). In fact, several of them even threatened to sue Musk if he decided to change his mind (he was doubtful because he thought that a significant chunk of the user base consisted of 'bots).
Later, he reported that had he not fired thousands of worthless and overpaid employees (mostly content managers), the company would have been two billion dollars in the hole. He also had to sell lots of things employees were wasting company funds on, like designer chair and coffee makers and even gym equipment (!).
After they were fired, many moved to decentralized platforms that were not only not earning much but was also filled with pedophiles. On top of that, censorship increased as they began to attack other other. Finally, the former head of Twitter who joined them in one platform, Jack Dorsey, eventually went back to Twitter:
Twitter/X co-founder Jack Dorsey left the board of Bluesky, an X rival, and even deleted his account earlier this week. He explained why in an interview with Founders Fund's Mike Solana.
www.entrepreneur.com
because of content moderation problems in Bluesky. He even added that they were making the same mistakes on that platform as they did in Twitter.
Eventually, some thought that X (formerly Twitter) would collapse because of the firings. Not only did that not happen it became even more efficient. They argued that Mastodon, Bluesky, and others would flourish, and they didn't. They argued that there would be more freedom of speech in decentralized platforms, and instead the opposite took place.
And many of them are giving up and returning to other platforms because of lack of engagement.
Meanwhile, it turns out that what X wants to do Facebook, Reddit, and others want to do as well, e.g., multi-function like retail, financing, B2B, video, audio, and virtual meeting services, etc., more revenue streams through subscription and charging third parties for accessing user content, and so on.
Why? Because there's not enough revenues from advertising, plus more users have found more ways to block ads.