The inherent problem with an AD support is that all champions have one form of damage that scales off of AD: their basic attacks. This leads to a situation where if they had some form of utility scaling off of AD, what's to stop them from going solo and just using AA to take down the opponent? For AP, it's a bit easier, as you have to build it to have it, and, while some AA scales on AP (at least on structures), it's tuned so that they don't deal that much with it.
I'm not saying that Riot couldn't ever make an AD support as a new champion, it's just that these are the current problems with one that Riot doesn't have a solution for.