I think it's got a lot to do with the mentality of the people - their attitudes had changed a great deal, since, say, the end of a 19th century. In the old days, people were raised with the notion of dignity and "reservation", as opposed to being kinda "relaxed" and "casual". If you take a closer look to the portraits (canvases) made by the Medieval painters, you will rarely encounter a smiling face. And you wont find ANY on the sculptures and paintings made by the Ancient Greeks or Romans - the act of smiling, and the humor in general, were regarded as something closely related to "low life". Thus, self-conscious people insisted on being displayed as such (dignified, stern, imposing) on canvases (then) and, later, on the photos... Now, since we live in the age of mass-media & modern democracy, those notions practically got eradicated - today's politicians, actors, entertainers etc. are greeting us with huge smiles on their faces, nobody regards the comedy as a proscribed genre anymore, etc. etc.