One difference is that Strauss-Khan’s DNA was found on the maid. In Duke case, the DNA on the stripper was from NONE of the Duke players, one of whom was not even there, IIRC. The source of the DNA was not made public.
The source of the DNA was not made public, most likely because the people (I believe there were three possible DNA matches, none of which matched any lacrosse player) were not on file. DNA can only be matched to someone - it cannot, of itself, identify them.