Why doesn't Pantheon's passive block single target spells as well?

Wondering if Riot ever stated why it doesn't do this. I'm sure its because he'll be OP from it but is there any numbers someone can throw at me?

Edit : Also this thread copied itself somehow. When I first posted it I got an image of Blitzcrank telling me something went wrong and just got a spam message.

Don't know what that's about but wth.