Wacom is a totally separate device from a laptop, and can be positioned so that you can comfortably write/draw while looking straight on at your screen.
It's woefully impractical for a laptop, specifically because it CAN'T be flexibly positioned where many people would want it. Plus, you'd have to lug an additional and cumbersome device around with you, occupy a USB port with it, and deal with a wire.
All of that makes the trackpad a much better option.
