Why? Having messages be iOS only, is working out perfectly for them.

Most iOS users get annoyed at the android users when they get green bubbles, instead of getting annoyed at Apple.

As long as that continues to be true, I don't see iMessage ever coming out for Android.

Apple has a pretty good market share and in their home market (because it's viable there, many other countries have insane prices) iMessage is compatible with Android users by SMS. This may be good enough, plus iMessage is just one of the features they sell with their devices. I doubt these cases are very comparable.