Seems like w the larger screen it would have been a no brainer.
If you read a lot into the Apple DNA there is something about design symmetry, if Apple decided to provide 5 apps in the dock it would also have provided 5 columns of apps at the same time. The option to make it zoomed shows Apple wants to evolve into the next software using incremental updates rather than step changes. They want to ensure that there is no learning curve moving from iPhones to iPhones.
Maybe the future ones might have that as the next logical incremental step.
My 2 cents...
The iPad allows for up to 6 icons in the dock, but still limits the display to 4 columns of icons in portrait mode.