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.
All posts on bsky are public and subject to use for training machine learning models. So a lot of people either don't care or are going to be very disappointed.