the problem with adding flight to old zones is that they weren't originally designed to accommodate players able to travel along the Z axis (i.e. unlike in 3.0 areas where they have invisible boundaries to keep you from going out of bounds, 2.0's areas were not designed this way), so first off, they'd have to redesign every single zone with new boundaries

that in and of itself could take a lot of time and resources the devs would likely rather put toward new content