Adding AoA indication to GA aircraft would be an unnecessary cost, and would further be pointless unless pilots are trained to use it.
By "trained" I don't mean somebody showing them how it works...I mean it's use would have to be extensively incorporated into primary training from the very outset. Otherwise it would be just another costly dust collector.
Unlike say a fighter, there are no normal phases of flight in which the AoA data would be interesting for GA aircraft...comply with the AFM speeds and attitudes and you'll never get near the AoA limit except maybe in the flare. Since it would not be interesting or useful, most pilots would tune it out...and it would be silly to expect them to suddenly revert to referencing the thing in a crisis.
It could be useful for aircraft certified for sustained FIKI. That's the only time I ever look at it.