There is actually quite a good article on the topic- Re-dressing Classical Statuary: The Eighteenth-Century "Hand-in-Waistcoat" Portrait

The simplest answer is that it was a fashionable attitude. In portraits and photographs you have to find something to do with your hands, otherwise risk looking awkward. This particular pose has roots in the Classical period, something immensely popular during the 18th century. It not only became one of the standard poses for portraiture, but was even used casually. Hogarth's Marriage a la Mode #4, a satire and commentary on the state of fashionable marriage, shows the French Fop reclining in that attitude as well as the portrait of the English magistrate. It was considered a gentile position, though clearly mocked a bit for appearing pretentious. Men have a habit of hiding their hands in their pockets if unsure of what else to do with them, an awkward and slumped forward pose. This posture would have encouraged a better appearance of the body, whether in still pictures or day-to-day life.