They simply don't get the point across. People don't want to spend a second more time than they have to read messages. If you want to get your message across, it must be memorable. Not surprisingly, the best headlines in newspapers are short. Pithy messages are essential because people have short attention spans, varying abilities to discern differences between products and a great need to put ideas, people and products into neat pigeon-holes.
People love to be mind readers.
That is, when you tell people you are, say, a writer, one could think you write novels. Another person could think you write computer manuals. A third could say you work for a newspaper. Therefore, you must be specific and not leave anything to chance. Here's an exercise I use in seminars and classes to help marketers get to the point quickly and accurately.