Because it's not important to ... anything.
The compilers don't care. The editors don't care. Back in the day, some operating systems didn't even HAVE "file extensions". DOS mandated them, DEC system mandated them. Unix didn't.
What's the standard extension for Fortran? For Pascal? For BASIC? Lots of convention, many system specific. But no standard.
You know what my file extension is for Lisp? It's .lisp. Well, a lot of legacy systems can't support a 4 letter extension. Guess it's .l then, or .lsp, or something else.
File names are local to the operating environment, and not necessarily portable. This is another reason the standard doesn't say anything about them.
Addenda:
I think there just needs to be some clarity here.
First, this is a retro site, so things need to be taken in the perspective of how it was and viewed forward, rather than using the lens of today and viewing backward.
Second, we're talking "standard" here. Standards are, roughly, "MUST". MUST do this, MUST do that.
1986 is just at the peak of the wild west of computing when things were really starting to settle down. There was a large diversity of systems, and things like a file extension STANDARD, a MUST, were not tenable. Today, while kernels vary, operationally modern computing is almost (almost) a mono-culture. Not so back in the day.
And, in the end, in the large, the language doesn't care. Extensions and conventions are a tooling issue. Modern things like Go and Java are more than just languages, they mandate a broader environment outside of just syntax and semantics. They're offering an opinionated approach to development beyond just the language.
Back then, the languages had to fit a variety of machines. Nowadays, the environments are bringing their machine with them.
We've come a long way in 35 years.