Why was Melchizedek the high priest of the whole human race?

Melchizedek served as high priest of the whole human race because he was a priest of the Most High God, a title denoting the God of all nations, not merely Israel. Since Israel did not yet exist, his priesthood encompassed all humanity and prefigured the priesthood Christ would later fulfill.

Les explains that Melchizedek brought bread and wine, which pointed to Christ's broken body and shed blood, even though Abraham did not understand it. He was not a priest of Judaism, Moses, or Aaron, but a priest of the Most High God. That term, as seen when Nebuchadnezzar called Daniel's God the Most High, refers predominantly to the non-Jewish world. Because the Most High God is God of all, and Israel was not yet on the scene, Melchizedek was high priest of the whole human race, a picture of the priesthood Christ would fulfill.

Whose high priest was Melchizedek?

Melchizedek was the High Priest of the Most High God, not of Israel or the Mosaic priesthood. Because Israel was not yet on the scene, he functioned as High Priest of the whole human race and pictured the priesthood Christ would fulfill.