Because after N scandals, they needed to make sure people would trust them. Meta had never considered itself bound to any promise or commitment they ever made to anyone (users, ads customers, etc.). But you want a monopole, you need to make sure people see no issue with using your services.

And they’re doing it again with Threads. And it works AGAIN, because they promised not to do anything evil. Pending the next inevitable scandal with users flabbergasted that Meta could have done it AGAIN.