The reason is quite simple I think. Turn your iPad into portrait mode. The icons are finger sized. Could they have squeezed a couple more in? Probably but with diminishing returns and increasing innacuracy.
I forgot the name of the jailbreak tweak. But they could have mimicked that tweak. It gave you the ability to add as many apps on the bottom dock as possible. It should you a certain number of apps by default on the dock. But you could swipe on the dock to other apps that were hidden initially. Seems they could have done something like this with no issues.