Mike:
Reason #1 is that I cannot get anyone to want to pose as crowning themselves....which is really unfortunate as it would make a great portrait! "When I rule the world..."
Reason #2 I think is largely due to the client's expectation. They often want a very specific look - the male wants to look successful, the female wants to look beautiful, and parent's often want children in idealistic settings more often than not, wearing nice clothes.
Reason #3 Little girls just look good in white dresses with bare feet. It is a bit like a piano standard - everyone wants to have at least one of those in their portfolio because they are just darn attractive and hit both the "expectation" and the "beauty" requirements.
My 2c.
