Mama is actually easier for infants to say than dada. ... However, "mama" only requires the lips to open and close. Infants have traditionally said "dada" first because while in the home with their mothers all day they hear talk about their father.

Our tongue naturally falls in a position resting on the alveolar ridge (same place for “d” sound), so it is a common first sound for children.

Ali Matisse, MS CCC-SLP