I mean the logic is obvious. If you are popular enough you want to keep the platform exclusive. If you aren't then it makes sense to go multi-platform.

BlackBerry thought their platform was strong enough and BBM would pull everyone else in. They were of course wrong.

It seems like Apple is strong enough to keep iMessage useful even if it is platform limited, and surely some people are buying iPhones for the status-symbol of Green Bubbles (or Blue, I can't remember I don't use iMessage).

Of course every platform wants to think they are strong enough to pull this trick, admitting otherwise is very difficult. Of course by the time it is obvious that you aren't strong enough it is probably too late.