I'm not really sure why other iOS 5 devices are being left behind for this Siri Personal Assistant feature, but many would like a port of Siri to the iPad. Maybe the iPhone 4S has some additional or improved hardware installed to enable this functionality? I know that once my iPad and iPhone 4 are converted over to an jailbroken version of iOS 5, I will be searching Cydia on my iPad for a way to try out Siri.
I don't know if Siri will ever make it to Cydia, due to it being software piracy technically. Cydia developers are already looking for ways to port Apple’s Siri technology to the iPad. iH8sn0w and Gojohnnyboi have somewhat ported a functioning version of Siri to a jailbroken iPad.