I can only guess, but here are my guesses:
As for why the edit suggested changing the quotation marks: plain text does not allow for proper (curly) quotation marks, so the
" character has become the standard to represent quotation marks in plain text and text-based contexts (as opposed to rich text) online. Granted, this part of the edit is probably not an improvement, because it's rare that a modern browser would have any problem with curly quotes, but given the ubiquity of the straight quote, it's no more than a marginal change.
As for why the edit was approved: since the quotes are a marginal change, the validity of the edit is mostly determined by the single most significant change, which fixes a piece of incorrect grammar. Therefore, the edit is considered a net improvement that doesn't introduce a significant problem and it gets approved.
But again, those are only my guesses.