Two reasons I can think of,
1) Many books and people the Bible refers to (such as the book of Wars and the Annals of the Kings) dont exist any more, so that ancient writers may have had more information on him that we do now.
2) You can gleen alot of information about Melchizedek by reading what is written about him but putting it into a list.
*sorry i hit the submit button*
Melchizedek king of Salem, brought out bread and wine. He was a priest of God Most High, and he blessed Abram, saying,
Bleed be abram by God Most High, Creator of heaven and earth.
And Bleeded be God most High who delived your enemies into you hand. (Genesis, 14:18-20)
what we know about Melchizedek is: He is a priest of God (showing that non-Levites can be priests,) which is importaint because hebrews show Jesus and High Priest of God which the Law wouldnt allow because he isnt a levite.
The author of Hebrews doesnt claim to know any more about Melchizedek then what the verse says, he was a priest and not a levite (nor even a Jew since they werent around then)