So they could not understand what He was saying because they could not hear His word. Now why could they not her His word?
In John 8:44 Jesus gives the answer for this; He said, "You are of your father the devil, and you want to do the desires of your father. He was a murderer from the beginning, and does not stand in the truth because there is no truth in him. Whenever he speaks a lie, he speaks from his own nature, for he is a liar and the father of lies."
The reason they could not here His word, is because they had murder in their heart, and therefore they could only hear lies. Jesus puts anger on equal footing with murder (see Matthew 5:22,23).