It requires concentration
Have you heard the phrases, "think before you speak" and "speak without thinking?" Those phrases exist because spoken language is trivial. Telepathic language requires substantial concentration. Curious consequence, it also means you're more easily distracted while trying to convey a message.
You also need to concentrate to hear it. Otherwise it falls into the background noise of life (see the volume issue I present next). Coming or going, it requires far more concentration than speaking verbally.
You can't shout, you can only direct
The wonderful thing about spoken language is that you can pick familiar voices out of a crowd, which means a person shouting in a crowd is more easily heard by those who recognize their voice. You have the same kind of recognition with telepathy, but all telepathy occurs as the same "volume." This means you can't shout to be heard, you must focus your message much more accurately. Curious consequence, this means telepathy is much less useful than spoken language when it comes to searching a forest for a lost child.
A way of manifesting this idea simplistically is that you can only communicate with someone you can directly see. No binoculars or other amplifying tools allowed. If you can't clearly see the target of your thoughts, they can't hear you.
It's like learning another language: and everyone speaks their own language
Most people think telepathy is like verbal communication: you can "hear someone speaking" without your ears. In reality, telepathy has everything to do with how you think, and the majority of our thoughts aren't words. They're emotions and reactions and associations and all kinds of things that frequently don't resolve into words. It can be thought of as an entirely different language with a few borrowed words from your native spoken language and a few more from whatever other languages you speak (like Pascal, C++, Fortran, and COBOL... wait 'til those thoughts show up!)
The consequence is that until you learn how someone thinks, you're "hearing" a foreign language — the equivalent of spoken sound that has no meaning until you've learned the grammar and vocabulary. It's theoretically possible to "speak" in the thoughts of another person (a rare and very difficult skill, and very personal) but much more common to eventually learn how to understand what they "say." Curious consequence: this suggests telepathy can be readily used as a covert language.
You're constantly having to protect yourself from the barrage
One of the biggest problems with telepathy is that you're living constantly in a crowded room. You can't simply don some noise-cancelling earphones, you must train yourself to push aside the mental noise. This effort is, itself, distracting and substantially limits how much real communication can occur (similar to concentration, mentioned above). In effect, it's like conducting every conversation in a busy night club. You're constantly fighting to hear and be heard. Curious consequence: this makes telepaths prone to nasty headaches.
Let's ramp things up a bit...
Normal people are afraid...
People are scared to death of both the advantages covert telepathic communication gives the gifted and the threat it represents against both society and the individual.
So they invented jammers.
Telepathy isn't magic. It's an honest-to-goodness scientific method of communication involving dark-matter sub-atomic particles named "Ondions" that vibrate in a mental field. Research suggests that Ondions are solely responsible for rational thought, for sapient life, for the soul, if you wish to express it that way. We do not think because we are, we think because Ondions are densely interwoven throughout neural tissue, giving the brain that magic oomph that allows for conscious thought.
And what's really cool is that Ondions are everywhere, and thought is thereby transmitted in a manner similar to radio waves. Ondions "vibrate" (if this concept makes sense, it's not a physical vibration but a state-vibration. It's as entangled as any other headache involving sub-atomic particles), meaning thought creates tremors in the Ondion-quanta surrounding us and it weakens as it propagates from the center of thought by the inverse-cube — which was expected, that being a common aspect of nature.
What trained people realized was, when within the interrupted quanta of a thought-generator, their own minds can sympathize with that tremor, allowing the two thought centers to synchronize momentarily. The brain interprets this as communication (but the possibilities are far more impressive than that).
Which was all nice and shiny until a means of measuring the Ondion field and its vibrations was published in the IEEE Journal of Communications and Networks, including a description of the devices invented to measure the Ondion quanta. An organization calling itself "FrEMT" (pronounced "fr-eh-mt," for "Free Exercise of Mind and Thought") then turned the technology into a jamming tech that can block all telepathic communication.
Then they learned that they could stop a person from thinking. When all the Ondions stop vibrating in the human brain, the human... stops. It's like the soul leaves the body and all that's left is the biological organism. It makes vegetables look smart.
And then they discovered that by modulating the Ondionic vibrations they could artificially broadcast an idea...
Reuters August 8, 2022
Tragedy struck Philadelphia's 30th street station today when hundreds of people waiting for the arrival of celebrated telepathic composer André Quittet suddenly and without warning simultaneously stepped onto the tracks before the decelerating train. Witnesses were traumatized by what they consistently describe as the eerie and silent way the victims simply "stepped forward." Investigators have been unable to ascertain any motive behind the apparent mass suicide and no connection between any of the victims other than their interest in Quittet.
Quittet, an accredited master of the emerging art of érzékek, known as "telepathic music" by most people, has always been a strong advocate of …