The optional iCloud feature called "Advanced Data Protection" is currently an opt-in. It comes with a significant drawback for typical pop and mom users --> If you lost you password and recovery key it's game over you loose everything. So I guess it's sensible to keep this as an opt-in until users are better educated about this drawback.
What will be quite significant is wether or not this feature will be available for chinese users.
It make sense from a technical POV to block ADP feature in poorly democratic countries that might request it like China and maybe tomorrow the UK.
PS : Once a significant % of users activated ADP it could be a good UX improvement to display a warning to mixed ADP status conversation that the conversation is not fully e2e encrypted. However this might be premature right now otherwise early adopters of ADP would be flooded by such warning.