mama?

How come, she doesn't say mama? She says dada, twenty times an hour, she can point him out on a picture and say dada. She can say duck, and does, a similar number of times a day. She can say book, and church and socks of all things but still no mama. I know she can make the sound but she just doesn't use it!!! She can do a snake's hiss, a sheep's baa, a lion's roar, a rabbit's nose twitch, a cat's purr and yet she will not say mama. Why is that?