The final paragraphs of the article answer your question, flyingmac:
...the passengers had not been correctly strapped in.
While the seats had shoulder restraints in addition to lap belts, it was difficult to put on the shoulder straps without a safety briefing and so only the lap belts were used.
The accident investigator said
'The impact was not severe. I would not have expected a fatality if the passengers had been restrained properly.