You really need this spelled out?
iMovie for iPhone is an app for editing video captured WITH THE device.
iPad has no ability to record video, hence no need for an editing application.
You *could* transfer movies to the iPad for editing. But this is cumbersome, and Apple does not do cumbersome (not if it can help it, anyway).
Now, if and when iPad gets a camera, or if and when a system for quick syncing of movie and other file content between iPhone/iPad is devised, be sure that iPad will get iMovie too.