Greater total contribution.

Because so much of tank sustain in 8-man content goes to waste (no reward in additional healer attacks, since they're already typically capped on offensive uptime), there is too little difference between a mid and great tank in total contribution from particularly good use of defensives to make up for the greater difference in total contribution between a mid and great DPS in damage dealt than between a mid and great tank.

As such, DPS more rewards your party for placing its skilled players in that role -- or, more rewards players for becoming skilled in that role (as compared to tanks or healers).