It seems to me that Fitz had enough stuff to charged Libby with the underlying charge .... and would have, but he knew it'd be laughed out of court. It's now pretty obvious that Libby (or anybody else, ftm) did not "knowingly" out a "covert agent".
So, Fitz took what he could get and call it quits.
Fitz kinda fizzled for the Dems. I'm sure they are deeply saddened. :)
She was not covert.