Why is the electric dipole moment of the nucleus of an atom equal zero?

Question

I read about the hyperfine interactions which cause the altering of the energy levels of the nuclues of an atom. Under the discussion about the electric monopole and quadrupole interactions, the term of the electric dipole moment interaction was determined to be zero. It was only stated that the nucleus has no electric dipole moment without giving any explicit reason. My question is, what are the assumptions leading to this result? It seems like the only assumption that was taken into account was the nucleus being a positive charge distibution and it does not supposed to cause the electric dipole moment of the nucleus to be zero considering the expression of the electric dipole moment. I would have assumed that some sort of symmetry of the nucleus charge distribution need to be considered as well.