Because there was a certain geopolitical event about two weeks ago that was a primary reason many still used Twitter/X and now that said event is over (and whose outcome will likely cause Twitter/X to further degrade) there is no reason to stay.
The exodus is not happening randomly or in a vacuum.
Or possibly, once the event was over, the users in question had the idea that Twitter/X affected the outcome in the way they didn't like and they decided to boycott Twitter/X?