Yeah they likely got a stash of money for it while the other CAs got nothing. First mover advantage I guess. It's not that they'd have been able to prevent it anyways, as the root stores are not under their control, especially when Let's encrypt is being supported, even started, by the entities which run the root stores (Mozilla, through Firefox, and Google, through Android). Ultimately it's the entities which run and deploy the root stores who have ultimate say on which CA gets to issue certs and which doesn't.