Historically, in order to promote battery life, Apple did not let the apps run in the background (until iOS 4.0 release in June 2010), but did provide this APNS (Apple Push Notification Service), which would let the app developers implement certain client functionality on their own third-party servers, which would then push the notification messages to Apple's central notification system (available since iOS 3.0 release in June 2009).
As such, anyone who would have wanted to write an app with any kind of background notifications at all during the whole iOS 3 availability window, would have had to have APNS support implemented, else, their app would have had no multitasking at all.
It would appear that this has never been the case in the Android land. Per my understanding, Google's take on APNS — GCM — Google Cloud Messaging — hasn't really taken off yet, probably because running out of battery in one day is not as evident as not having the background-notifications functionality at all, and the APNS/GCM architectures require the app developers to themselves run the extra servers on their own side, which is obviously not free, either, and would certainly require extra engineering and complexity, for achieving something that's not entirely obvious to the naked eye.
Additionally, according to a question on StackExchange, GCM is a second incarnation of Google's take on APNS, where the first one, now discontinued, was called C2DM (Cloud to Device Messaging Framework). It would appear that C2DM had quotas, which were then abolished with subsequent GCM. The exact dates on the introduction of GCM and C2DM are not clear, however, their device-side parts would appear to be backwards compatible, since they both claim to support Android 2.2 and above. Android 2.2 "Froyo" was first made available May 2010.