Why is Siri only for iPhones and iPads and not for other Apple products? Will Siri ever get released for other platforms, such as OS X?
Yep. Ben S is right.
It's just what Apple has decided. Apple is good at drawing a distinction between their desktops mobile OSes, so they probably don't see a need to have Siri on OS X.
It makes more sense on mobile, anyways. You don't really need instant info hands-free on a computer like on a phone.
We can't predict the future, so who knows if OS X will get it for now? : )