Why did Jesus teach in parables about the Kingdom of God?

Parable, a form of illustration, was used by Jesus extensively in his teaching because it accomplished his goal. It was, and remains, a very effective teaching tool.(Matthew 13:34-35) . It :
1. holds the hearer's attention
2. gets us thinking
3. stirs our emotions
4. aids our memory and
5. works in any culture, at any time period.
Teaching with illustrations (mental pictures) helps people get the sense of what's being taught, as it puts things in terms they are familiar with (ie: God is like a "father"(Luke 15:11-32), Jesus is like a "shepherd"(John 10:11-15), Faithful worshipers are like "sheep"(Luke 15:1-7), False religious leaders are like 'blind guides' (Matthew 23:24). It also:
6. motivates right-hearted ones to take action, and eliminates those who really have no interest in truth, as they can't be bothered to take the time to question things they don't understand(John 6:54, 60-66)(Mark 4:33, 34)(Matthew 13:13-15, 36)(Matthew 22:15-21)
7. provides a quiet, but clear, rebuke to those who need it, without opening up a clear line of retaliation. (Matthew 9:11-13)(Luke 7:36-50)(Luke 10:30-37) and
8. makes things out of our realm of understanding, understandable (such as the Kingdom of God(Luke 13:18, 19),(Matthew 13:3-8,18-23),(Matthew 13:24-30, 36-43, 47-50)(Matthew 13:31, 32; Mark 4:30-32)(Luke 17:6)(Luke 11:5-8)(Luke 13:1-9)(Daniel 2:31-35+44). As humans, we would have no way of relating to things in the spirit realm, unless someone put them in terms we were familiar with.