It has nothing to do with viewing angles. Take the U.S.S. Voyager for example. That model looked like crap from pretty much every angle. It has to do with how easy it is to film the model. And in this case, flatter is better. That's why ILM's models such as the Reliant, the Excelsior, and the Enterprise-D were all much flatter and more compact than the TMP Enterprise.