There are reasons DET isn't best and SS is, job's DPS rely on certain things that set it aside from other DPS jobs. The reason SS is best for BLM is because the faster you get your fire Is out the faster youre going to get your free fire III proc (firestarter i believe it is called). Same theing goes for BRD, the more often your DoT ticks hit a crit the more often you're going to get a free bloodletter proc. For SMN and SCH the more often your pet crits the more often you get a 20% boost in SS. So there are little gimmicks to each job that pushes one of these attributes above DET. DET is generally the best secondary stat for an overall sustained damage/healing increase.