It’s because it’s one of the only things that effectively can hit Israel.

They release the submunitions at much higher altitudes than they were intended so they spread across a much larger area and thus ineffective at hitting anything other than an urban target.

But on the plus side for the Iranians they separate outside of the interception envelope of even the exo atmospheric interceptors Israel has so they actually get through even if each sub munition is only a nuance at best.