Basically it's a Markup Language, not a data transfer language, so it was far from ideal to use it as one. See for example the ambiguity between what should go

Basically it's a Markup Language, not a data transfer language, so it was far from ideal to use it as one. See for example the ambiguity between what should go into tag attributes vs. tag content etc. Also the eXtensibility makes things too complicated for many use-cases. You don't have to care about any of this with JSON.