While only guessing, it would seem more likely that Apple knows they do what they are doing, which keeps most people's data safe and private (and people who don't use icloud backups can have complete privacy), vs if they encrypt all backups, FBI will make sure to remind the public and congress more loudly everytime they can't catch someone because of it, and then much more likely they congress will ban encryption completely resulting in far less security and privacy for all users. With so many people moving to Telegram and Signal, that might end up happening any way, but what they are doing would simply seem a way of avoiding/delaying that.