>I never understood why they weren't part of the original spec

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

>Trailing commas in objects were only introduced in ECMAScript 5. As JSON is based on JavaScript's syntax prior to ES5, trailing commas are not allowed in JSON.

json was largely `eval`d when it first hit the scene. It was easy to parse because you just received it and `eval`d it directly to parse. worked in every browser. security concerns over time lead to this being rightfully replaced with the JSON object and its associated encoder and decoder.

however, since early javascript didn't allow for trailing commas, neither could JSON if it wanted to be able to be `eval`d.