Mark borrowed these two points and a lot of the structure of this sermon from Don Carson's very helpful sermon here.
Jesus tells parables because in line with Scripture (Isaiah 6), His message blinds, deafens, and hardens (v 10-17)
Jesus tells parables because in line with Scripture (Psalm 78), His message reveals things previously hidden in Scripture (v 34-35, 11-12, 16-17, 51-52)