Because it's supposed that a lot of passengers, especially those in steerage, returned to their cabins when it was clear that all the lifeboats had left. The bodies recovered were, by and large, those wearing their lifejackets and standing on the Boat Deck when she sank.
All the engineers, and most of the "black gang" (greasers, stokers etc). perished below decks and they make up a considerable number of the crew lost.
The controversy over the number of women and children in steerage who perished comes down to several factors. Firstly, a significant number did not speak English, so it is presumed that some never even understood the instructions they were given. Secondly, steerage passengers were held behind gates for some time after First and Second class passengers were allowed up to the Boat Deck. Some have taken this to mean that the steerage passengers were deliberately held back from an opportunity to save their own lives by the crew, but given the apparent confusion amongst the crew as to just how much danger the ship was in until the final minutes, this may not be a fair appraisal.
For years, the assumption that she went down intact meant that the popular image was of Titanic sinking by the head until she was almost vertical, then slipping beneath the waves. With what we know now regarding breakup on the surface, something that as far as we know was not predicted by Thomas Andrews (on which the evacuation plan was based), it's possible that the crew ran out if time long before they thought they would.