Hebrews was [and is] not regarded as sufficiently ‘mainstream’ either in the academic circles or in its ‘market’, the churches, to warrant much attention.
Then as now we knew that only the Gospels and Paul were really worth bothering about! In my undergraduate days the rest (Hebrews, James, 1 & 2
Peter, 1, 2, & 3 John, Jude and the Apocalypse) were referred to as ‘the backend’ of the New Testament. But at least then the back-end was an integral part of any reputable university’s basic New Testament syllabus, which cannot be said of today. With the reduction of the Biblical Studies component in most theology degrees, and its concomitant, the reduction in time and content of our syllabuses, Hebrews – along with a number of our New Testament works – has effectively been made redundant, and we are left with a ‘canon within the canon’.