Apparently since Apple made it more convenient to iPhone users to not only message other iPhone users from iMessage, but also receive their text messages in that single app too, it's a monopoly?
That probably didn't help things but I think the root cause comes down to the usual source of monopolies in this case: having vertical integration and a default app. Technically you don't even need an iPhone or SMS service to use iMessage. It is by the most common use case though, and one which increased the amount of integration a bit.