It's interesting because I was pondering this point after coming back from a convention yesterday. I think Bizzaro is correct about people feeling as if they "have" to do a certain kind of magic to win competitions. However, I really feel that it's more about the fact that there is a LOT of magic. When you have, say... 3 billion card routines and 2 routines with, say, penguins, it's going to be more rare to see an act with a pet penguin.
Also, a lot of the magicians who are competing at conventions do it to learn. Some are finessing acts or looking for feedback, so it's not always the most innovative material that you see there. It's often something that the magician hopes one day will turn into that after performing it so often.