My only guess is that they developed a proprietary system and as a business want to maintain control of their proprietary system as one of the selling points to their hardware ecosystem. It makes them money, and a business in a capitalist market, they want to keep their competitive edge.
Which, honestly, I get. Imagine you created something (and note, not invented) and someone decides you should be forced to share it simply because it sells better than what the other guys have.
My only argument against this is that there already are internet based end to end encryption messaging systems in place, both private and FOSS. It's not like Apple has a monopoly on this type of technology or system.