Via The Hungry and Foolish:

I’m not convinced this will come true, but if it does, I’d have to wonder what the motivating factor behind this decision is. In my opinion, iMessage is a reason many people stay on iOS. If it went multi-platform, it would make it much easier for users to switch to Android.

In keeping with Apple’s big security push, putting iMessage on Android would make not only any Android user’s messages presumably more secure but, more importantly to Apple and their customers, any iOS users they would be communicating with as well.

Delivering a similar but naturally less consistent experience to Android users might have the opposite effect to what many believe. Rather than drive users away, seeing what iMessage offers and how much better it is than most of the offerings on Android could actually entice some to switch to an iPhone. Extending an olive branch will also go a long way in changing the mindset that many Android users have towards the company.

In addition, with Apple rumoured to be adding payments to iMessage, there’s an opportunity to push that service into the hands of many more people. Apple Music for Android was a clear sign that Apple is willing to bring services to Android if they see a reason. With iMessage, it’s not nearly as cut and dry but reasons do exist.

Update 1

Speaking with an executive within Apple, Walt Mossberg seems to have gotten to the bottom of the whole thing:

When I asked a senior Apple executive why iMessage wasn’t being expanded to other platforms, he gave two answers. First, he said, Apple considers its own user base of 1 billion active devices to provide a large enough data set for any possible AI learning the company is working on. And, second, having a superior messaging platform that only worked on Apple devices would help sales of those devices — the company’s classic (and successful) rationale for years.

Makes sense to me.

Update 2

I may be wrong but I'd swear we've heard this info about Apple's detailed Android iMessage mockups around WWDC this year, some versions closer to Material Design, others closer to the look of iOS. Interesting that it's floating back into the news cycle after all this time.