You don't have to wear a seatbelt in a vehicle which was never fitted with them when it was first manufactured. (So, for example, you can drive around in a 1960s mini without wearing a seatbelt). Any new vehicle registered in the UK has to comply with the multitude of Construction and Use Regulations (which includes having seat belts fitted). Similar rules apply across the EU. But the two Popemobiles were both registered in a non-EU country (Vatican City State) so the EU regulations don't apply to the vehicle and,because it didn't have a rear seat belt fitted when it was manufactured, there is no obligation for the Pope to wear one.
Chris