Changing something like that would actually be quite a significant change to the game, and it would likely cause Riot to have to rebalance every champion. In general, Critical Strike affects your autoattack, which scales off of your AD no matter what kind of champion you are playing. So AP based champions do get critical strikes when using their autoattack. Most AD abilities are also unable to get a critical strike, with some exceptions. But those exceptions generally have limits.
I personally believe that adding critical strikes on AP abilities, or even most AD abilities, would greatly overpower the champions.
I could be wrong though, it is an interesting idea.