I'm sure they've been well aware people have wanted it for a while. My best uneducated guess is that iOS 7 has been such an overhaul that it demands all the team's resources right now, and that a quick reply functionality baked into the OS would take some time to develop well and test for bugs, so it's on the back burner while they just perfect iOS 7 for now. I'm hoping to see it in a later build but I'm not too optimistic.