I think "can't" is "wont" in this case. Apple law enforcement guidelines state:
"All iCloud content data stored by Apple is encrypted at the location of the server. When third-party vendors are used to store data, Apple never gives them the keys. Apple retains the encryption keys in its U.S. data centers."
and
"iCloud content may include email, stored photos, documents, contacts, calendars, bookmarks, Safari browsing history and iOS device backups. iOS device backups may include photos and videos in the Camera Roll, device settings, app data, iMessage, SMS, and MMS messages and voicemail. All iCloud content data stored by Apple is encrypted at the location of the server. When third-party vendors are used to store data, Apple never gives them the keys. Apple retains the encryption keys in its U.S. data centres. iCloud content, as it exists in the subscriber’s account, may be provided in response to a search warrant issued upon a showing of probable cause."
So in short, Apple retains encryption keys for much of the data (including iOS backups), and will provide decrypted data to US law enforcement on request.
https://www.apple.com/legal/privacy/law-enforcement-guidelin...