The witness may be traumatized and only got a glimpse of the perp, that would make it even harder.
I have often wondered if they show the witness pictures of different eyes, noses, mouths and other features and let the witness select the ones that look like the person as they remember him.
The obviously did not do that in this case.
They pull out transparencies of eyes, noses, mouths, etc.
The composite is made from pieces that match the descriptions.
Or at least at some places.