It's called that because it was written as a letter to Hebrews. Most of the Epistles are titled after the group they were written to. (Corinthians was written to the Church at Corinth, etc.
From http://christianity.about.com/od/newtestamentbooks/a/Book-Of-Hebrews.htm
Date Written:
Hebrews was written before the fall of Jerusalem and the destruction of the Temple in 70 A.D. Written To:
Hebrew Christians who were wavering in their faith and all future readers of the Bible.
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