When it comes to Android vs iOS, we see the same sort of thing each and every year. Where Google announces big changes and such during their Google I/O conference, Apple follows with many announcement inline with those of Google's at their WWDC conference. While the functions and end results that Android and iOS ultimately display are very similar to one another, they exist in their own ecosystems. For users on Android, it's a Google world that they live in - with some flexibility thanks to other apps and app stores - and for iOS users it's very much Apple's world that they're living in. iMessage has long been one of the few features that Android users have wanted to see hit their platform, if only to communicate with those too cool to use WhatsApp, Hangouts or some other cross-platform alternative. During WWDC, one Apple Exec has spilled the beans on just why iMessage will never be available on Android.
Walt Mossberg, writing for the Verge, details a conversation he had with a "Senior Apple Executive" that told him iMessage would never be coming to Android due to two key reasons. One of those is the ever-so-unsurprising reasoning that keeping iMessage on certain devices - like the iPhone, iPad and Mac line - would help sales of those devices. While iMessage might not be the deciding factor for some, it's just one more example of Apple using a key feature to try and either keep loyal customers just that or to coax new ones into their infamous "walled garden". The second reason is perhaps a little more surprising than the first, being that the 1 Billion active devices that iMessage is in use on will give them ample research for their Artificial Intelligence - think Allo - that they're working on.
For iMessage, there's no need for Apple to expand it to other platforms, the service on its own doesn't provide Apple with any extra revenue and those that really want to communicate with everyone, regardless of chosen platform, no doubt already use WhatsApp. For now, and the foreseeable future, it appears as though Apple Music will be the only exception to the proven rule of Apple not sharing their apps or services outside of their own "walled garden" which many Android users will be totally okay with.