Apple can nickel and dime developers because a good app on their ecosystem is profitable.
Nobody is making enough profit via the Twitter API to make it worth staying on. Twitter is a distant #3 social network that itself has never really made money.
This is being too generous. Facebook, Instagram, and TikTok are all far larger than Twitter. Reddit is also just barely larger than Twitter, though at that scale you have to consider who has more bots. Twitter claims slightly more MAUs than Snapchat, but since Snapchat isn't a viral network it probably has no bots to speak of, making it larger than Twitter in terms of real users.
Nobody is making enough profit via the Twitter API to make it worth staying on. Twitter is a distant #3 social network that itself has never really made money.