(1) Business who are familiar with web tech and for them it's simply good business to not invest in entirely new (for the dev teams in there) stacks and data formats.
(2) Political pressure -- we know that NSA successfully pushed a broken crypto once in the past, so it's not a stretch to theorize that influential people were whispering in the right ears at the right time.
Even though #2 is just as likely as #1, please note that I think #2 probably happened much, much less than #1. We should never attribute to malice that which can be attributed to incompetence and short-sightedness.
EDIT: There are several very good candidates for data formats out there which are much more efficient than JSON. Adoption is seldom an issue; if one corporation pushes for the format, it's a matter of several months at the most before that format becomes widely supported. Replacing JS might be much harder but efforts continue even today.