I would also like to know the true answer. But first, I want to remind you of something that I'm sure you've noticed yourself. If it's generally a reasonable principle in life that you shouldn't reinvent the wheel, then among programmers - and especially in the open source community - the opposite is true: "This wheel that guy invented is really cool. I just wonder why he made it out of wood. You'd need an axe. And I hate axes. Why not make it out of clay? I have some clay lying around anyway and nothing better to do at the moment."
I guess the same is true here. All programmers I know hate XML. I always loved XML because it's nothing but text, but in a form that both humans and machines can understand easily. But for the programmers mind it's to verbose. (Strange that we don't use machine code any more).
This resistance to an implementation in XML and that to the majority JSON "feels better", may be the only relevant reason.