For a very long time monitors were used only and entirely through cables and speakers on the deck of the stage. But then the advent of in ear monitors came about. So the thing is this: In the real world we cannot isolate sounds from one part of the stage to the other–there is such a thing as stage noise and it is a beast. But also in the real world we have radio.
Radio is one of the things that brought along the advent of the audio engineer. For a long time music was seen as an art. Those who “mixed” — which in the old days meant actually mixing, not using Compressors, and Gates, and PEQs, and Delays, and Reverbs, and Side Chains, ect.– were seen as artists. Those who studied music saw them as lesser or inferior to “real” musicians.
Then it all changed rock and roll came about, particularly The Beatles. Geoff Emerick was the sound engineer of The Beatles and he changed the way people think of audio guys. Suddenly people saw audio people as both artsy and scientific. The science side came about here and they began to engineer things that were not just artsy, but started to design things for radio. This began the embracing of radio and music. This lead to change.
This all brought IEMs into the world we live in now. The idea of having personal monitoring in headphones on the stage due to small portable radios that were held by the musician on stage was ground breaking.
At the time Physics was primarily looking to space, and designing things to be smaller and smaller so they could go to space. Radio was starting to be seen as an archaic form of communication, but electromagnetic communication was starting to take off. Small computer were being built, the cellular phone was being thought up, and wireless household phones were being thought of.
In music…nothing was really changing at the time.
Then someone named Jerry Harvey created the in ear monitoring system. He took what was being done at the time in physics with radio, and he took what was needed at the time with music on stage and he smashed them together and created the first ever IEM in 1995. This. This is the future, and it was!