Without going into too much detail, I can tell you a huge challenge with encrypted backups is compliance - many people simply do not opt-in to using encrypted backups. It's a difficult line to walk because ultimately you don't want to pester users too much about using encrypted backups either. In case it's not clear, the reason it requires opt-in is because you (i.e., the user) control the encryption keys and therefore you must take some action to write down a key or passphrase. As you may imagine, the average messaging app user is not as tech savvy as the average Hacker News user and many simply don't want to go through this extra step. Even worse, if they opt-in to encrypted backups and lose their keys, they will end up blaming Meta for not giving them a way to restore their message history when their phone gets lost (it's not easy to explain that it's your responsibility as the user to keep your keys safely stored).