why isnt there an ad support champ

Supports are used for their utility, and utility items to have AP, tank stats, and/or cdr.

AD scales into the late-game (unless you build armor pen, but that's more AD assassins than AD carries), which is detrimental to the support role. Supports generally don't scale, they don't need to.

What would an AD support do? The best we got is Ashe, with slowing aa's. Their auto damage will fall off if they don't get fed (which supports shouldn't need to), their spell damage will either have to scale hard with AD and thus be abused in solo lane roles (like an AD Lulu) or be low enough that you won't bother buying the 1300 gold BF sword (because your first 1300 gold will likely be spent on a support item and sightstone).

There's really no point in an AD support, not to mention we'd need to have AD utility items and take a good guess which classes are going to abuse that just like midlaners abused FQC.

No AD-themed support items, because AD items that also offer utilityewould get abused by Assassins and Marksmen the way Mages abused

That would require new items, AKA more work.

Because in general AP is cheaper and most utility items are AP

I've seen people play Ashe, who has her minor in support, as support. But very rarely.

They had that splash of a Blitzcrank 2.0, co-designed by Ziggs, with all kinds of rockets and bombs. Maybe he'll be an AD support, if he's 1) a real thing and 2) ever happens.

There are a few AD champs that can support. Poppy is very good as support right now. Panth is great for a kill lane. Trundle is very good at high elo. Volibear can support in some comps, and Shen support has been good for a while. Shaco, Kennen, and Thresh are hybrid supports who can build AD.

The second you make an AD support, you get the Yasuo effect. Yasuo was meant to be an AD mage and look how that went. Same thing would happen to an AD support.

That and support don't get alot of gold from first