Papers want to sell more copies, simple as, especially in the current climate where online is in the ascendancy. They will choose whatever picture will help achieve this.
I have been part of many news desk/picture desk conferences where this has been the case. I've also witnessed editors deliberately choosing the most unflattering image of, say, Camilla Parker-Bowles when there were some really lovely ones available, just because that would fit better with their readers' perception of her as being ugly, horse-faced and incomparable to 'Saint' Diana.