Why does the shell output often use `mixed_characters' in its output?

Question

I see backtick ( ` ) (also called a grave accent) characters mixed with apostrophe characters ( ' ) used together in all kinds of command-line output. Surely the reason/history behind why is documented online somewhere, but I couldn't find where.

Here are a couple examples of what I'm talking about:

From the make man page:

If makefile is `-', the standard input is read.

Some rake output:

.../ruby_koans/koans/about_strings.rb:6:in `test_double_quoted_strings_are_strings'

Why the inconsistency?

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I suppose the broader question here is "Why not use 'proper' single or double quotation marks, as appropriate?" but I realize that the apostrophe (and the grave accent) are simply more available on "standard" U.S. keyboards. Why that is, though...)

Answer 1

This had already been asked and answered on Programmers (now deleted). The top answer was a best guess that it came from LaTeX.

A similar question has also been answered on SE English Language & Usage. The top answer was that it was to work around limited character sets, which lacked separate characters for ‘this style of quotation marks’.