In the design of any new model or miniature, one of the key things designers have to come to grips with is keeping the "theme" of the army intact with continuity of design. That is, each new miniature must fit, style-wise, with the rest of the army.
Imperial vehicles are blocky and inelegant. Ork vehicles are downright brutish and ramshackle. Eldar and tau vehicles are both curvilinear and elegant, but with different elements that clearly distinguish them. While some of the vehicles shown below can fly in space, they are all supposed to be able to fly in an atmosphere too. Right.
I know...I need to get over it.