Why couldn't you put the app on an iPhone or iPod touch ? Or in a simulator on MAC ? It's not like the device is essential. It might be more practical for some purposes, but not essential.
Because of the form factor. iPad is close in size to the papers used before. iPhone is way too small and a Mac is way too big and clumsy. Also a Mac has a clamshell design as opposed to being just a tablet that fits nicely in front of the pilot.