I suspect $$$ is the reason for no STC--it takes a lot of that commodity to get through the STC process--and I don't think it's required.
I don't profess expertise in this area, but there are many things which can be permanently mounted in certificated airplanes without STCs. My IA has plenty of expertise, I rely on him to follow the rules, and he has a good relationship with the FAA rep he has to deal with. My airplane has a full page single spaced of mods, some in place when I bought the airplane but most installed by him with the blessing of the FAA obtained in advance when necessary. I would agree that the Dynon EFIS isn't acceptable in a certificated airplane, but comparing it to the AOA because both are instruments is sort of like comparing apples to onions--they're both ball shaped and edible, but that's about it.
Zane, it's true that I don't even consider the AOA in most circumstances, only when coming into a very short strip. I tend to fly "seat of the pants" without reference to either it or the ASI most of the time, because I've been flying for a long time and I really do know my airplane. But when I want to really slow it down, it's a valuable benefit. Just like one doesn't use a handheld GPS as a primary nav instrument in IFR conditions but instead uses it to augment situational awareness, the AOA offers that benefit. As for a scan issue, mine is mounted on top of the panel so it's easily seen with peripheral vision without taking my eyes off the approaching runway--no need to scan it. BTW, I hardly ever look at my VSI--its lag makes its usefulness much less than the primary instruments of altitude and airspeed. When I have inexperienced passengers, I do use the VSI to confirm my "seat of the pants" feeling that I'm not exceeding a comfortable descent rate that will bother their ears.
In any event, I don't regret having the AOA installed, at all. Whether others choose to install one in their own airplanes is their business--but it's not their business to discourage others. After some 50 hours or so of using mine under all conditions of loading from pilot only and a few gallons of gas to full gross, airport elevations from 1200' MSL to 7800' MSL, and landing areas from very short to almost 2 miles long, I would like to see one in every light airplane, because I think it would materially reduce the approach-to-landing accident rate.