I'm guessing this is because they haven't figured out a way to do it yet. I'm not very well versed in how these systems work but surely this type of encryption can't be disabled by Apple remotely (or they would have that backdoor they don't want)?
> Customers already using Advanced Data Protection, or ADP, will need to manually disable it during an unspecified grace period to keep their iCloud accounts. The company said it will issue additional guidance in the future to affected users and that it does not have the ability to automatically disable it on their behalf.