Holy cow, I wrote a book!
When I discussed years ago
why operating system files tend to follow the old 8.3 file name convention,
I neglected to mention why the old MS-DOS filename convention was 8.3
and not, say, 11.2 or 16.16.
It's a holdover from CP/M.
As I noted when I
discussed the old MS-DOS wildcard matching rules,
worked hard at being compatible with CP/M.
And CP/M used 8.3 filenames.
Why did CP/M use 8.3 filenames?
I don't know.
There's nothing obvious in the
CP/M directory format that explains why those two reserved
bytes couldn't have been used to extend the file name to 10.3.
But maybe they figured that eight was a convenient number.