The wording of your question is a bit strange (we usually don't say that bonds have IMFs and ionic compounds don't consist of molecules), but the answer to your question has to do with bond polarity. Strong IMFs are associated with very polar bonds (because increased polarity means greater magnitudes of charges at the poles and, hence, greater electrostatic attraction). Since ionic bonds involve the transfer of entire units of charge (to form cations and anions), they are highly polar (and highly polar means strong IMFs). Polarity in covalent bonds, however, does not arise from transfer of entire units of charge. Rather, they involve unequal sharing of electrons, and thus partial charge. Covalent bonds are, therefore, not as polar as ionic bonds (and less polar means weaker IMFs). I think it would be a good idea to keep in mind, though, that your question should be more along the lines of "Why are ionic bonds more polar than covalent bonds?".
