The cognitive leap to associate moving a mouse or keyboard (in your hand) with stuff going down on a screen is significant compared to just touching something you see and it moves. The former represents how we typically interact with a computer, the later how we interact with everything else.
It's actually kind of amazing that Apple made both those interaction types - Apple 2 was THE FIRST time you had a keyboard + screen, and the iPhone/iPad are(perhaps more arguably) the most successful implementations of touch screen interaction.
I'm actually quoting Steve Wozniak, but sorry, I should have made clear I meant for an affordable consumer device. Apologies.
Edit: No doubt, to be totally honest it's not an area I really know too much about, I just have Steve shouting "THE FIRST" engraved in my memory from a few years ago...
It's actually kind of amazing that Apple made both those interaction types - Apple 2 was THE FIRST time you had a keyboard + screen, and the iPhone/iPad are(perhaps more arguably) the most successful implementations of touch screen interaction.