It's mentioned alarmingly often in historical texts because it was and always has been really extraordinary. Diplomats have served rulers and nations for millennia, and they frequently have been members of a ruling family or some other person with close ties to the leader. This is, incidentally, why it's not really a bad thing for anyone involved when a US President appoints a close friend or a big campaign donor to an ambassadorship. It means he's a guy with a direct line to the leader. Professional civil servants handle the hard work anyway.

But I digress. Envoys and diplomats have frequently been, if not powerful themselves, then power-proximate and usually stood to benefit personally in a variety of ways if they succeeded in whatever their diplomatic objectives were. Okay, once in a few generations some ornery Greeks threw a few Medes down a well, or a Khan thought a guy was being a little rude, but really diplomacy is one of the most important functions of any ruling body, and for thousands of years rulers have known it's a bad idea for it to break down.

You have to remember that the role of an envoy was for much of history a very very powerful one. They had to be empowered to negotiate on their ruler's behalf, to strike deals, offer concessions, and even to make credible threats. There was a Roman consul during the late Republic named Popilius Laenas who managed to prevent a war between the Seleucid Empire and Egypt by showing up at the war camp of King Antiochus's invading army, where he handed him a decree passed by the Roman Senate that essentially ordered him to stop being a dick. When Antiochus said he would consult his advisors, the Consul drew a circle around him in the sand and demanded that Antiochus reply to the Senate decree before exiting the circle. Antiochus was so intimidated by the power of Rome that was being represented in this one individual that he stood down his army and went home with his tail between his legs.

Diplomats represent the power of the nation they serve. As such, for most of history, they and their safety has been of great import to their home country and to the country they have been sent to negotiate with. The concept of diplomatic immunity is used today mostly as a plot device in bad procedural crime shows, but its history is ancient. You don't kill or imprison diplomats. Violating such a long-standing accord is like refusing safe harbor to ships in distress. It's happened plenty of times, but it's worth commenting on.