It is far easier to express oneself than to communicate. You can express yourself to a rock just as well as to a human being. But to communicate, you must understand how your words will be processed by another mind. You must estimate their level of knowledge, their potential for misunderstanding, their emotional response to your ideas, their capacity for new ideas, and their willingness to listen.
As social animals, we evolved to optimize our communication by relying on non verbal feedback in one on one interactions. When communicating in writing, the challenge is doubled, and doubled again when ongoing feedback is not provided. No wonder then that so few people learn how to be effective writers.