I thought it made sense when Episode I came out; you can see the Empire and the vehicles used in it as something that is done for more of a functional purpose, whereas the vehicles from the Republic were able to find a balance between form and function.
Eh, why not? 1. The Empire Strikes Back 2. Tie: Revenge Of The Sith 2. Tie: A New Hope 4. Return Of The Jedi 5. Attack Of The Clones 6. The Phantom Menace