Originally it was because iMessages were free, while SMS were $0.15-$0.20 each. So glad Apple broke that monopoly, along with many other anti-consumer wireless provider restrictions.
Did Apple really break that monopoly? WhatsApp was released 2009 with the explicit pitch that it was "free SMS". iMessage launched 2011, and with the anti-consumer Apple lock-in, and isn't even much used outside US.
Also free/unlimited SMS was prevalent in many countries before WhatsApp or iMessage. And there were plenty of IMs before that too.