Full names on envelopes and the convention someone mentioned earlier about it being Mr and Mrs on the envelope and Jill and Jack in the letter or card is because of the division between the formal public sphere and the informal private sphere.
In public, the man is historically in charge (remember couverture?) and so on the public part of the correspondence, the envelope, must always be man-first. In some periods of history, men opened all their wives and minor children's letters so the name on the envelope wouldn't necessarily affect who opened it.
If the letter was formal public sphere stuff, like a bill or legal document, it would open with a man-first greeting with Mr and Mrs.
BUT, if it's a personal letter, like a family letter or greetings card from friends, it's a private item and so the woman, whose domain is the domestic and family private sphere, should be the first name in the greeting and the writer should use first names.
Also the surnames on the envelope ensure that if you misaddress the card to 12 instead of 21, the people in 12 will realise that the letter is not for them, which they might not do if the envelope is addressed to "Mum and Dad".