Why is the dipole moment directed from negative to positive ?
Why is south directed to north? Or the other way? This has no meaning in physics.
Because that way the direction of the dipole moment vector is aligned with the direction of the external electric field. If you look at a rod that is charged positive at one end and negative at the other you are usually not interested in the internal field present inside the rod. So it makes sense to base the definition of the dipole moment on the external field.
Separate names with a comma.