Because the BA "boffins" were trying to waitlist you into a class (D) that doesn't exist on the majority of North American AA segments. As you may have figured out by now, a DONEx can book into "A" class on 2-class North American flights, it's in the OWE rules. And as you probably already no, there is very little "Business Class" within the US, on almost all flights, the front cabin is marketed and sold as "First Class", hence only A inventory, not D. Exceptions are 3-class flights that offer both First and Business (primarily only JFK-LAX/SFO) and flights to Canada/Mexico/Caribbean, where the front cabin is marketed and sold as "Business". Don't ask why.