Why did the Pharisees not understand Jesus' speech?
They could not understand because they could not hear His word, being of their father the devil rather than God. They rejected His divine origin, disbelieved His virgin birth, and accused Him of being born of fornication, so His words did not register with them.
Les explains that in John 8:41-44, Jesus told the Pharisees they do the deeds of their father. They responded that they were not born of fornication and claimed God as their one Father. They did not believe in His virgin birth; the story going around Nazareth was that Mary and Joseph, even if Joseph was the father, had conceived out of wedlock, and likely that Joseph was not even the father. So they threw at Jesus that He was the result of fornication, an immoral act. Jesus then said, if God were your father, you would love me, for I proceeded forth and came from God; neither came I of myself, but He sent me. He asked why they did not understand His speech and answered His own question: because you cannot hear my word. It never registered with them. He then said they were of their father, the devil.
Why do ye not understand my speech? even because ye cannot hear my word.