There probably is a story behind it. Like some developer looked into the bug in the beginning (when when the load time was not that bad because the json file was smaller), saw that the time was spend parsing json, and then, without thinking too much whether this makes sense, wrote in the bug ticket that, to make it faster, they would have to switch from json to something better/faster/different.
Since switching the file format is probably some work, maybe involving many differnt teams and network protocols, the bug was closed because it too much efford.
And then, whenever somebody complained, the old bug was referred which unfortunately never got a proper analysis.
Sad, but I saw this happening many times in the past in some projects. Since the old and wrong analysis will be in many powerpoint slides and heads, it is incredible hard to argue against or get even time to work on such a problem. Usually, you need someone who just takes some time form other projects without telling anyone and ninja fixes the problem to solve it.