Why not spell lycopodium with a k?

Greek wrote lykos with a kappa. Latin had no habit of using k, so it wrote c instead, and English took the word from Latin rather than from Greek. That single decision, made a very long time ago, is why we have cinema and not kinema, cycle and not kykle, and lycopodium and not lykopodium. A c in front of o is always hard, so the pronunciation is never in doubt. Only the spelling is.

A c that has to be read as k, because the letter behind it is a Greek kappa.