For example, we need to allow quoted keys in objects. But if we already have a quoted syntax, there is no need to support unquoted keys as well. Likewise, JSON knows only
"..." double quoted strings, single quoted strings
'...' do not exist.
The result is that it's really easy to implement a JSON parser (maybe 20 lines with a nice parser generator). We wouldn't want to throw that advantage away (which helped become JSON so accepted. Contrast with the complexities of parsing XML). Of course, a production-ready implementation should be lenient in what it accepts.
Could a change like this be retro-fitted into the standard? No, because backwards compatibility. Feel free to write a JSON implementation that accepts unquoted keys, but you can never emit such a string, in case the receiver uses a stricter implementation. And because you generate all your JSON via a library instead of writing it per hand, adding those quotes isn't any extra burden.