Why would they say to all new users, that they cannot have Advanced Data Protection, whereas older customers can?

Now you have a certain percentage of users with encrypted data, and a certain percentage of users that do not. The UK government will not like that. And now Apple has shown that it will not take a stand for privacy it might have to do it to comply.

Ah, you missed the part where Apple also said existing users will have to turn it off at an unspecified date.