On the other hand, its because it fucking sucks. It's the largest company in the world (by market cap) which has a revenue of a third of a trillion dollars every year. They can afford to make it free but they don't for their own gain. All their competitors make their myriad of chat apps (that only Americans don't seem to want to use) free and available on as many platforms as possible. The only real reason Apple doesn't is because they want to hoard more and more of their money and become an even bigger company. That just sucks, it's shitty, it's worse for the world. They have hundreds of billions of cash reserves that they don't even know what to do with. I don't give a shit if it's their right or whatever to do it, I still think it sucks and is worse for everyone who isn't a VP at Apple.
Open standards are nice, decentralization is nice, having options and choice and cross-platform things are nice. Having a gigantic company make a choice to create a silo where they're the only ones allowed to use it is not nice.