I believe the main reason is that you have to be connected to the Internet in order for Siri to work. This is easy with the iPhone but not so easy with the iPad which has many wifi only models. When these units are away from a wifi connection Siri would not work.
No matter how hard you try there would be a sizable crowd of wannabe geeks claiming "Epic Failure" on Apples part because they could not get Siri to work in some one stop sign town that has only one wifi network and it's in the one room library.
Except that the new iPad has dictation, which also requires an Internet connection. No, Apple didn't put Siri into the new iPad because they wanted it exclusive to the iPhone 4S for now; Apple makes more money per phone than per iPad. Siri costs extra money for the bandwidth and servers that it uses, so Apple will wait until the perfect moment to introduce it to other devices.