Because they have fewer keys, Cobra. Keyboard space is at a premium on a laptop. Anything not between 'esc' and the lower right contol key don't have a home in the space alloted to the keyboard on a laptop, and so the absolutely vital keys like del, home, end, page up/down, and the printscreen key get placed in any spare space that can be squeezed in. Forget the numpad. A lot of this is resolved by use of a fn key that doubly-maps certain keys, obviously.
The question is and will remain, why, with the premium already being asked for each key, did something as rarely (read: never) used as SysRq ever get its own key when it doesn't even get one on a full keyboard.