As we know now, the application-related policies are quite different for macOS and iOS. If macOS still makes it possible for users to install programs outside the App Store, lower security settings to allow unsigned applications, and create programs not incorporating App Sandbox, all this is not possible on iOS without jailbreaking the device. Thus, the common penetration vectors differ for these operating systems.
As the App Store infrastructure is quite well-protected against malicious apps, especially because of the obligatory signing with quite expensive certificates that can be promptly revoked, and this way, deactivate the corresponding threat on the vast majority of the devices, mass malware authors rarely follow this path. Still, there are some exceptions to this rule, for example, when malware authors get access to stolen certificates or inject malicious functionality into legitimate software. An example of this could be an XcodeGhost threat that managed to get access...