Apples and oranges.

One is „accept a wide array of syntactically malformed input, ill defined, substantially complicating the parser“, the other „accept two trivial extensions to the standard that are widely used and don’t affect the resulting data in any way.“

So far nobody has tried to get trailing commas into the standard AFAIK.

Comments were explicitly disallowed from JSON since the beginning, because they could be used as a side-channel for vendor-specific extension mechanisms and harm interoperability.

> accept two trivial extensions to the standard that are widely used and don’t affect the resulting data in any way

That's always how it starts. If they're truly trivial the reasoning also cuts the other way. Remove them upstream.