Why can't I use passbook on my iPad?

Passbook not for iPad. Only for iPhone and iPod Touch.

http://support.apple.com/kb/HT5457

Why would you want to use a device as large as the iPad at a kiosk, when you already have the iPhone which you say that you carry with you all the time anyway? Maybe the Apple software engineers thought the same thing so they decided not to include it on the iPad.

Demo wrote:

Why would you want to use a device as large as the iPad at a kiosk

What? You mean you wouldn't want to bust out a 10" tablet at the airport to scan your boarding pass????

because I, and many people, have iPad but not iPhone. Your assuption (and I guess Apple's too) is just inaccurate for us who had paid for a device that capable of doing this but been left out with such a lame excuse (user experience > functionality?) Well, I really can't find how logic talk out of it could be any helpful.

Then tell Apple how you feel

http://www.apple.com/feedback/

Only they know their motivations, however given that the phone and touch have passbook and greater photo options I can see their train of thought being that the more portable devices need apps that the larger devices don't.