Why did Jesus call the Pharisees and Rabbis "descendants of those who murdered the prophets"?

He didn't exactly call them that...he said that THEY called themselves that. "And YOU say, "If we had lived in the days of our forefathers, we would not have taken part WITH THEM in shedding the blood of the prophets". He was merely citing another example of their pride and self-righteousness...that they honored prophets and righteous people who were killed by their own ancestors and had the audacity to suggest that they wouldn't have made the same mistake as their forefathers.