In older galactic wars, when lightsabers are prevalent on both fronts, it was the norm for militaries, private security forces, bounty hunters and other adventurers alike to arm themselves with weapons and armour protected by a cortosis weave. So cortosis is not exactly rare, since it can be used in mass manufacturing by the war industry and is accessible to common use by private individuals too.
Since the Separatists are preparing for war with the Republic and its Jedi guardians, and the Sith plan to use the Clone Wars to help wither the Jedi down by attrition, why aren't lightsaber resistant troops more prevalent? I would expect super battle droids or at least more elite droids like destroyers, commandos, magnaguards etc to be so.