The ripper got away with his murders because he killed prostitutes. It is almost impossible to trace the last houses of a prostitute. Murders of prostitutes are always difficult to solve.
Not to mention the fact that there wasn't usually sufficient interest, by the police, so solve murders of prostitutes. Just the sheer number of murders committed by the Ripper scared the middle and upper classes sufficiently that they put a lot of pressure on the police. But even that pressure couldn't produce actionable evidence.