It is because you need the aaa command to specify which method are you going to use for accesing the device, for example, you are connecting via ssh, you will need to add the following command:
aaa authentication ssh console LOCAL
Local keyword goes all in uppercase. That way, when a user that is on the local database (command username cisco password cisco privi 15) will be able to login to the asa using the assigned username and password.
Mike
Mike