Dear student,
Locomotion and movement are not considered as the defining property because movement is also shown by non-living object like we can move a train from one place to another. It is the intentional movement and locomotion that is shown only by living things, and not by non-living things But since moment is defined as the change of state from rest to motion or vice-versa hence intentional movement/locomotion does not fall under this category. Hence, we can say that locomotion and movement is not the defining property of the living organisms.
Regards