I think it comes down to the basic function of the two devices, iPhone and iPad. Often when you use your iPhone to do the things that Siri let's you do, it's interrupting something else. Whether it's saving a reminder, setting an appointment or sending a text, Siri let's you "get in and get out" to deal with those little interruptions quicker so you can get back to what you were doing. It makes a device of convenience even more convenient.
But the iPad is different. Apple doesn't think you're using your iPad while doing other tasks like walking down the street or socializing with friends. They think you're sitting in a comfy chair or at a table and doing stuff with it. When you're using your iPad what you're doing is using your iPad. If you need to do the things that Siri let's you do, just press a few buttons and do them yourself. You have a minute or two to spare if you're using the iPad the way it was meant to be used.
Ultimately, the iPhone is a traveling companion, but the iPad is a destination. You don't need Siri's help to get where you already are.
