Armchair CEO rant incoming;
I remember reading Issacson's biography on Steve Jobs in which they spoke about iTunes being on Android. He said they were ready to develop it and he scrapped it eventually because "Why do I want to make Android users happy". In a sense I think it was wrong because it's a service that makes money for the company so furthering it to other platforms is a no-brainer. But in the case of iMessage, why does Apple want to make Android users happy? Where is the revenue for iMessage, ya know?
Over the years I think the "blue bubble" unintentionally became something popular. Just look at memes online regarding "green bubbles" and you'll see what I mean. Apple is probably better off keeping iMessage as iDevice exclusive as long as it has luster.
THIS.