I don't mean to debate the reason itself as to why women have visibly developed breasts. The point is there's no clear reason, and thus there doesn't need to be a clear or logical reason why Au Ra have breasts by extension. (Plus the original question was posed on the assumption that Au Ra are reptiles / scalekin... which is not true.)
Again, even if you want to argue that they are, it's probably a design choice stemming from how humanoid shapes are easier to identify and sympathize with. Even though Warcraft's draenei are blue-skinned people with horns and ungulate legs, they still have a basic human shape (and yes, the females have boobs) despite coming from a different world altogether. A human form is just easier to identify with... regardless of whether or not it makes sense. (Compare a race of aliens from Star Trek: Starfleet Command whose name I forget, but they had three arms, three legs, three sexes, and breathed methane. Not so easy to relate to, eh?)
This comes back to the visibly developed breasts of human females being mostly fat and connective tissue. (And for the love of Hydaelyn, Au Ra are NOT related to dragons in any discernable way despite what appearances might suggest!) The mammary glands that produce milk are beneath the breasts, believe it or not, not inside of them. Humans are also the only mammals who display permanently enlarged breasts in females; the mammary glands and breasts of other mammals do enlarge during pregnancy and while nursing, but otherwise females remain similar to the males of their species in that respect.
Saying "Well they're mammals, obviously they'd have boobs!" isn't enough. (As much as I wish it were.)
As for why female lalafell appear to lack a significant bust? Well, aside from the "attracts attention" hypothesis, the only other leading hypothesis on why human females have permanently developed breasts is that it helps infants suckle. Seeing as lalafell faces are kind of flat, I doubt it would be an issue for them. (Design-wise lalafell are supposed to evoke young children regardless of how physically mature the lalafell in question actually is, so it makes sense from that angle as well.)