Why are london forces weakest intermolecular forces?

London forces are weak forces because  their origin is due to a transient or instantaneous dipole which in turn depends other factors such as:

1  Polarizability of the atom or molecule: Polarizability is a measure of the ease with which the electrons and nuclei can be placed. Hence  greater the polarizability, greater is tha magnitude of instantaneous dipole and sronger are the london forces.

2 Size of the molecule; More is the size, more is the strength of the london forces because larger sized molecules have diffused electron clouds hence their are greater chances of distortion to form instantaneous dipoles.

3 Number of electrons; More is the number of electrons, more is the chances of distortion.

So we can say that london forces are weak forces.