First and foremost to avoid accidents: We don't want a misconfigured HTTP website accidentally opening a new Excel window for every fetch/XHR call or to have to

First and foremost to avoid accidents: We don't want a misconfigured HTTP website accidentally opening a new Excel window for every fetch/XHR call or to have to fight Excel defaults to get JSON to open up in our IDE or Dev Tools of choice. We don't want random shell scripts accidentally curl-ing things to Excel. Things like that. Secondly for "ownership" reasons: We don't want to give non-developers the mistaken impression that Excel "owns" JSON and that it is a Microsoft format. I've had people tell me that CSV must be a Microsoft format because you can double click them in Excel and they show an Excel-like icon (in some ways it has been too long since Lotus existed and Excel was in the "we'll take all of our competitors' file associations too" era). On the one hand it might be nice to blame all of CSV's problems on Microsoft and Excel if that were actually the case, but on the other hand it also confuses people as to the real/valid uses of the format. Unfortunately, too, that transitive relationship goes both ways and I've heard second hand that CSV files are among the reasons the Excel team hopes to never add another file type association again because supposedly they get far too many support requests for CSV file problems that maybe shouldn't be their job to deal with. A separate file association adds some intent of "this file was actually meant to be opened in Excel and hopefully the developer actually tested it some".