Cheaper yes, but there was a whole lot of interservice political pressure going all the way back to the 1980s to diminish the Marines role in Embassy security, and covert ops in general.
This goes back to some political issues covered in SOF even during the first Gulf war when a politicized SF unit "liberated" the US Embassy in Kuwait city for publicity purposes, doing something like $100K in damage to the place clearing some rooms by grenade and automatic rifle fire, only to find graffiti from a Marine Recon unit that had been there, checked the place out, locked the doors back up, and left under orders to not officially "clear" the embassy because some "specialized unit" was supposed to be doing it. That unit was led by John Rambo shortly before he was appointed to be the Sgt Major of SF and he discussed it in a documentary like it was some major battle. There was zero enemy opposition. I knew Rambo's younger brother when I was in the Marines and he would brag up the achievements of his older brother in SF.
Prior to that, there were some high profile cases engineered to point toward Marine Corps incompetence in handling embassy security, along with a lot of the training regimen for embassy security staff being sabotaged from within. A good friend of mine went to the embassy security school in 1989, and reported back that they were doing 1950s style police method training, isosceles grip on .38 revolvers, lame shit like that. Lots of spit and polish, protocol, skills worthless to actual security. People were complaining about being ordered to act as potted plants in uniform. Meanwhile, some select groups of Army people (mostly SF) were getting the real training resources and being shifted into the real security positions in plainclothes, with Marines just being the uniformed potted plants at the gates.
The real State Department security people go through their own training courses now which are usually held at federal law enforcement training facilities and if they are military, have to be approved for leave or a suspension of military service to do the diplomatic security jobs. The give back has been that they usually get around double what their normal military pay would be.
