Hi Shawn,
Curious question for sure. It is listed as 'not preferred' because of the fact that m14 is not listed as a 'first choice' size in the grand list of 'preferred basic sizes' - ref section K16.6 of the Genium Publishing Drafting Manual. M14 is listed as a 'second choice' however, with M12, M16 being the 'first choice' sizes.
As for not recommended, that's another story. I've found that often an M14 bolt is the best choice in certain applications.
...now where was that grain of salt?...
hope that helps
Paul R.