Because the iPhone is a capacitive touch screen and the Modbook resistive? Resistive screens are capable of pressure sensitivity and tend to require a stylus. A drawn line gets wider the harder you press. Multi-touch gestures are difficult to pull off. Capacitive screens work with fingers and work well with gestures. However, there is no pressure variation – all iPhone drawing apps just put down a single width line. The one app that has varying widths does so by guessing the velocity of the drawing and then mimics as best as possible what size line is being drawn.

Do you envision a device with two screens?

Why not both, a ModBook style and an iPhone style tablet in one?
>_<