Why doesn't sudden impact grant adaptive penetration instead of giving both MPEN and lethality?

This rune is way better on hybrid champs like Katarina than it is on every other champion that it was designed for. Gunblade is already insanely gold efficient on both Katarina and Akali, and this 1 rune is more responsible for pushing her over the top than anything else.

I'm actually really surprised it Sudden Impact isn't adaptive considering every single other rune has been made to give adaptive stats, and sudden impact is by far one of the most powerful early game runes.