That makes sense for a lot of reasons. The first reason is you don't have someone's cocker spaniel walking around on the stage looking terrified and pulling focus from Annie singing probably the most well-known musical theatre song in the world. The other reason is, it allows you to put another kid in the show. Granted, you have to tell some little girl who had dreams of playing Annie that they have to play a dog, but hey, it is show biz!