There isn't enough time in the day. The leader doesn't have time to explain a task, assignment, or responsibility. The leader doesn't want to communicate an unpleasant message. The leader is afraid of the response, doesn't want to hear it or deal with it. Sounds silly, but if you never initiate the message, it will never be received.
People tend to be close minded to new ideas.
People are preoccupied with family concerns, other work, pressing assignments, relationships, transfer, etc. Differences in personalities. If someone doesn't like you, they may decide not to listen to your message, thoughts, or ideas. As a leader, you need to watch and be aware of the expressions of those in the audience to be sure they are taking in what is being said. Once again, this sounds easy, but think of all the pitfalls.