One reason was taboo. People did not want to talk about suicide and, when they had to, used composite expressions like the medieval lawyers' "homicide (or felony) of oneself," or others, tinged with euphemism, like "to hasten death," "to die by one's own hand" (to quote from over a dozen expressions in classical Latin). A second reason, linked with the first, was an enduring difficulty in understanding the character of suicide. This is best seen in the history of a much older word, the Greek biaiothanatos. It literally meant "violent death" and included suicide along with deaths we would classify as accidents, like falling from a bridge, all of which were thought polluting, and had consequences for burial and for the dead person's status as a ghost. Although the Latinized successor-word, biothanatos, came increasingly to be confined to suicide, it still carried the old ambiguity as late as the twelfth century. The slowness of that discovery is confirmed by early legal texts, where rabbis, Christian canon lawyers, and commentators on the Koran all, in their turn, insist that it is intention that distinguishes suicide—as if people did not know.
So the mere linguistic history of suicide already reveals a lot about how it was conceived. It suggests, further, that the taboo extended beyond the topic to the act of suicide, however vaguely distinguished. Religion absorbed the taboo, and in time gave it force and rationale. Already in Greek antiquity we find indignities attached to the burial of suicides, so understood. Similarly, as ideas of hell became better defined, we find suicides assigned to it, first (this time) in India.