Answer:
Due to a larger number of electrons in the heavy atoms.
Step-by-step explanation:
Spectral lines are caused by the emission of light by electrons when they transit from a higher energy state(excited state) to a lower energy state.
Hence, the more electrons an atom has, the more the emission and spectral lines. The less the electrons an atom has, the less the emission and spectral lines.
Therefore, heavy nuclei (which contain more electrons) such as Iron will emit more light and so will have more spectral lines than light atoms like Hydrogen and Helium.