The State Dept pukes don't like using Marines. Healthy, horny young men in a country where alcohol and fooling around with the local women are forbidden pastimes, will find ways to get into trouble and embarrass the suits.
Then again, when the jihadis are coming over the walls, the "local security" disappears down an alley, and all you have for security are any armed Americans.
For the suits in DC, having armed Marines presents an additional problem: they are more likely to open fire rather than allow themselves to be meekly slaughtered.
Marine embassy guards are among the most professional Marines. It is well known in the Marines that they make grade the fastest of any MOS. Off duty behavior is strictly controlled and there is no faster way to get sent home.
However, all Marine embassy guards are under the direct tactical control of the ambassador or charge d’affaires. If they wimp out the Marines cannot open fire unless fired upon and that may be too late.