Most Android users I know hate Apple for irrational reasons, just because they are Apple.
So now you'd have to convince Android users to download a new app, sign up for an appleid, and install iMessage.
And even then, I doubt it would hook to the phone number like it does with iPhones, so you'd have to start iMessage chats with people using their appleid email addresses. Or the Android user would have to start the conversation.
I don't think the majority of my friends; who are all mostly Android users, would go for such a thing.
Now the SMS synchronizes between my mac, iPhone and iPad, I don't really care if people have iMessage or don't, and I just don't see Android users totally ditching their current SMS apps so they can hook into Apple's ecosystem, when they are trying to avoid it in the first place.