Imagine you want to write a JSON parser and prettyfier. Then writing the keys out in the same order as they were inserted would be very important. Yes, it's true that key order doesn't matter to the computers that consume JSON but humans consume JSON too and key order often matters to them.
You may be able to tell from this reply that I've never built a JSON parser and prettyfier :-) but I'd guess that in this case, I'd never have any reason to ask for a bit of the JSON "out of order", so I'd just use arrays...?
