With a core collapse type II supernova the core remains, compressed into a neutron star and the visible spectrum results from the heating of the outer layers of the star, mainly hydrogen, hence the strong Balmer lines.
I guess that still leaves though the question as to why 1c, core collapse SNe where the H and He envelope has already been stripped away, do not show strong Si II lines. This review paper though does suggest it perhaps is seen weakly in 1c spectra (fig2)
https://arxiv.org/abs/1908.02476
