The iPad and desktop apps are siblings – both require a minimal horizontal resolution of 1024 points to run. The 12.9" iPad offers 2732 x 2048 px. (1366 x 1024 pts.) which is just enough horizontal points when ran in portrait. The 10.5" iPad is 2248 x 1668 px. (1124 x 834 pts.), running out of points when used in portrait.
The Phone app is a different beasty altogether.