That's not necessarily specific to iOS. Certificate pinning is usually done inside an app, not at the OS level. An app can choose to ignore the system certificate store and, for example, pin the cert used to talk to its private API. This is possible both on iOS and Android.
iOS is even easier than Android to add system certificates and can be done without rooting or jailbreaking the device unlike android. cert pinning is done by the apps not the system.
Android apps could also do certificate pinning with the same effect though? In this case there isn't any difference between Android and iOS in functionality.