Why do the spectra of the molecular species show broader bands compared to atomic spectra such as that seen for the hydrogen atom?

Explanation:

The  atomic spectrum is more like the transitions of electrons among electronic energy levels in an isolated atom. Atomic spectrum is mainly affected by interaction of the transitioning electrons between the nuclei spins and between the other electrons in the same atom.

However, molecular spectrum involve transition of electrons in molecules with two and more atoms(it can be in the same or different). Also, the valence electrons are now in different orbitals than that of  the atomic orbitals and the orbital structure has changed the electronic transitions are different

The molecular spectra have been broader than the atomic spectra due to intra-atomic transitions.

The molecular spectra have been consisted of transitions of electrons in atoms of a molecule.

The atomic spectra have been composed of the transitions in between the atom's energy level.

The atomic transitions have been resulted within in an atom from the lower energy level to the higher energy level. The multi-atomic transitions results in the broader spectra in molecules, while atomic spectra has been narrow and sharp.

For more information about molecular spectra, refer to the link: