Why did Abram pay tithes to Melchizedek in Genesis 14-20?

Nobody knows for sure because The Bible does not say. All the commentaries and tithe-preachers say that he was either commanded to so or else freely chose to do so.

A better suggestion comes from 14:21. The law of the land all around Abram's world of 2000 BC taught that one must give spoils of war to one's local king-priest. That is what I believe is the context.

Why? The king of Sodom asked Abram for the persons because he knew that rightfully Abram could keep them as slaves If the law of the land controlled the 90% of 14:21 then it also controlled the 10% of 14:20.