I am not sure I would say that it is out of simplicity that you do not put "regnant" after HM Queen Elizabeth's title.

When you have been THE Queen for nearly 70 years - everybody on the entire planet know who you are - and know that you are THE Queen of the UK. And therefore it would be total nonsense to use the word regnant.

Could you ever imagine 2 ladies standing in a queue waiting to see Queen Elizabeth actually saying: "oh, do you see the Queen Regnant is wearing a beautiful pink suit today?" Of cause not! nobody in their right mind would refer to her as the Queen Regnant. Nor would they of cause ever be in doubt if she was a Queen Regnant or a Queen Consort