She's probably the most comfortable around you, and completely used to you already. Your voice is probably more soothing, whereas someone else's voice is new and more "ooh!" to her, if that makes any sense. One day it'll occur to her that .Mommy is funny

and she'll decide that she only wants
you to sing her special song, because no one else does it right. She'll only want
you to fix her afternoon snack, she'll run to
you for kisses when she falls and scrapes her knee, and will want
you reading her bedtime stories because you do the voices just the way she likes them.
I'm going through the same thing with .Alyvia and her father. She tolerates things with him that she won't with me. But she won't let him put her to bed - only .Mommy will do. So I take some comfort from that, and try to remind myself that at the end of the day, she only has one mommy and she knows that.