Part of this complex issue, as mentioned below, is that especially in the past, an unwed mother was viewed as a disgrace, an embarassment, to be hidden and scorned and otherwise looked down on. The reason for all this being that she went with the lively demands of her own body or heart (and/or her seducer's) instead of remaining rigorously observant of the morals of her Victorian or Puritan society.
Hell hath no fury like an arbitrary moral code scorned, you know.
If this social disapprobation were NOT in play, there would be far fewer abortions.