“That is why.” The Greek is dia touto, and it connects the parable which follows with the forgiveness in the verses above. Jesus was teaching on unlimited forgiveness, and after making the statement that Peter should forgive seventy time seven times, he said that is why what we should be doing can be compared to the Kingdom of Heaven, because God practices unlimited forgiveness (cp. Meyer’s Commentary on the New Testament and New Testament Commentary: Matthew, by W. Hendriksen).
“a certain king.” The certain king in the parable represents God, who will one day in the future have a Day of Judgment and settle accounts with people. In Christ’s parables, the “king,” and often the “man” or “landowner” represented God.
[For more on the “king,” “landowner,” “ruler,” or “man” in Christ’s parables being God, see commentary on Luke 15:11. For more on Christ ruling the earth in the future, see Appendix 3, “Christ’s Future Kingdom on Earth”].