Apple has been known to reject apps from their iOS App Store because they’re too similar to features already built into iOS. Siri seems to be one feature they are particularly defensive of. Apple is quick to warn developers who are submitting Siri-like apps not to mimic native features of the OS.
Jordan Kahn for 9to5Mac:
…Apple wants an app’s features to remain “distinctly different from the iOS behaviors and interfaces to avoid causing user confusion.” Developer Sparkling Apps reached out to let us know Apple rejected its latest app submission called “Voice Answer,” with Apple telling the developer the Wolfram Alpha-powered alternative is “too similar to Siri.”
Apple isn’t rejecting all Siri competitors, as Sparkling Apps has an app called “Voice Ask” on the App Store that is in the top charts in the Reference category. “Evi” is also still in the store. Both apps use the True Knowledge database, while some of the rejected apps use the same Wolfram Alpha knowledge base as Siri.
The Siri competitor/substitute apps allow users of devices that can’t run Siri to access Siri-like features. Siri is currently limited to the iPhone 4S, so there is certainly a market for apps that are allowed by Apple. Apple’s reasoning for rejecting some Siri competitors while allowing others is vague at best, with Apple saying the apps rejected do not comply with the “Guidelines for Using Apple’s Trademarks and Copyrights”.