Apple knows if they released iMessage for Android they'd lose customers. Simple as that really. There's much more cons than pros for Apple to let Android users use iMessage.
I disagree. First, people don't buy an iPhone for iMessage. They buy it because it is a well-designed package that integrates well with their Macs.
Second, nearly everybody I know uses other messaging apps (Facebook, mainly, but also various others) instead of carrier-provided text messages, sometimes because of $$$, but usually because of integration with their daily workflow. MMS and SMS are basically moribund at this point, which means iMessage is really only a selling point if all your friends use iOS (which would be unusual). Otherwise, you'll likely be talking to most of your friends in FB anyway, so why not just talk to all of them there?
IMO, Apple seriously blew it with FaceTime and iMessage. They were miles ahead of everybody when they introduced those products, and they had the opportunity for their technology to take over the world and to make the product better on iOS, but available everywhere. Instead, they got greedy and tried to keep it entirely for themselves. And because all that end-to-end crypto has no value if 60% of your recipients will get their message via an SMS anyway, the rest of the world moved on without them and came up with third-party cross-platform products that filled the niche and are usually considerably better on Android. Now, almost nobody I know uses either iMessage or FaceTime on a regular basis.
Apple's chance to use these products as a competitive advantage has long since come and gone. At this point, the only thing left for Apple to do is to make it possible for third-party apps to write custom Messages plug-ins in iOS and hope that they can at least convince all the third-party messaging services to make their services available through a nice, unified interface so that you can see your messages all in one place. Otherwise, that whole "Facebook can send/receive SMS and MMS" thing is going to eliminate the very last plausible reason to use Apple's messaging apps at all.