The character sets used historically with Unix, including ASCII, don’t have a tick character, so it wasn’t used. As far as I’m aware no common usage for that character has been introduced since it’s become available; nor would it, since it’s not included in POSIX’s portable character set.
` was apparently originally included in ASCII (along with ^ and ~) to serve as a diacritic. When ASCII was defined, the apostrophe was typically represented by a ′-style glyph (“prime”, as used for minutes or feet) rather than a straight apostrophe ', and was used as a diacritic acute accent too.
Historically, in Unix shell documentation, ` was referred to as a grave accent, not a backtick. The lack of a forward tick wouldn’t have raised eyebrows, especially since ' was used as the complementary character (see