Why can Hebrews be called a sermonic Epistle?

Although called (in full) the "Epistle to the Hebrews", Hebrews has none of the characteristics of a letter, other than some travel plans and the final verses which might have been added in later times. If, as it seems, Hebrews was never intended as a letter, it was very likely written as a sermon. Thus, it can be called a sermonic epistle.