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.
Siri hasn't been included in the latest beta version of iOS 5, and may instead be a cooked in especially for the iPhone 4S owners. If that is the case, the simple task of enabling the feature may not be possible through Cydia.