Because apple's products has a capacitive screen, it doesn't have pressure sensitivity like the N900 has (which IS utilized in MyPaint), plus capacitive screen must use a special conductive stylus, while the N900 and its resistive screen supports any tool which applies a required amount of pressure.

I'd say that the best screen for a graphic artist is a combination of a resistive screen (for pressure sensitive input, which is important), and capacitive screen (for accuracy) - but if a graphic artist has to choose between a capacitive or a resistive screen, I think he'll go for resistive, and as far as I remember all the Wacom tablets, which are the favorite tool for graphic artists, are pressure sensitive...