Why is Nullifying Orb only effective vs magic damage?

I mean, I expect the counter-argument to be because then AD Assassins that need to face dive for kills would get cucked, but what about ?

Why can't it just be a shield vs the primary damage type of the champ that triggered it like 's shield?

You can see in the buff bar that a champion has this rune, and it's something you have to consider when playing an AP assassin, so I think it would be fair for the same to have to be true for AD Assassins.

Edit: Noone has mentioned this yet but I just realized every ADC would then take this. Pretty sure this is the reason it doesn't block physical damage. I personally think this is a good candidate for the ol' melee only tag then, since as someone mentioned, it's designed for those with weak laning (and imo riskier champs, which tend to be melee).

Edit 2: Apparently it procs on non-champion damage and blocks that damage (which would mean if it blocked physical it'd block tower shots). So... remove that... That way it only works vs champs like I kinda assumed was the intent.