Single quotes are quite a common English punctuation character. Particularly in the strings I tend to hardcode in programs, it is way more likely that I need to write a single quote than I do a double quote.

When that happens, I have to escape the quote (if the language even allows escapes in single quoted strings) or change the whole string to double quotes.

Because typing a single quote will break parsing, it's also hard for tooling to assist. If I put a single quote unescaped, the auto formatter will probably barf.