Ultimately, people decide what is and is not punctuated. Also, in BrE, when an abbreviation ends with the final letter of the word it abbreviates, in BrE there is no need to add a full-stop, so Mr is not incorrect in BrE. What is driving the other changes (approx) is, I believe, quite simply the fact that people write their own letter nowadays and find it easier to save the keystrokes. It's being driven by pragmatics- it's easier not to type the full-stop than to type it; whether you are for or against it is another question, but language will always define itself. My father dictated his letters to his secretary, but I write my own; I think that is the driving force, for better or for worse.