Why do *nix folk like to use ``...'' rather than the " character?

The real reason (and what may be the root cause of the LaTeX usage) is that many pre-unicode unix fonts (both for the console and X), and two common Adobe Postscript encoding vectors, had typographic opening/closing quote glyphs at these positions, so 'this' would look like ’this’, and ``this'' looked like ‘‘this’’ or ‛‛this’’, which was (especially in a proportional font where these were only 2-3 pixels wide) as close to typographically correct as you could get back then.

This goes back even further to typewriters, where the ' glyph would often be set at an angle to accomodate its use as an overstriking acute accent.

See also:

• http://www.cs.sfu.ca/~ggbaker/reference/characters/
• http://www.cl.cam.ac.uk/~mgk25/ucs/quotes.html

One reason is probably the influence of T_EX to the Linux world. In most flavors of T_EX, the sequence `` creates a typographically correct opening double quote (“) and '' (two single quotes) or " create a typographically correct closing quote (”). So When you type

``Why do you always say `shut up' to me?''

It actually yields

“Why do you always say ‘shut up’ to me?”

using T_EX. (Perhaps with better kerning, though).