Thing is, the intelligence is the main factor and orcas have two ways of killing sharks. They can bite (with teeth up to 7" long and force of 18 tons per square inch) and they can ram (a common defense tactic against sharks that members of the dolphin family use). Sharks have only one way of killing and they are very succeptible to paralysis after being rammed and flipped upside diown. Especially by a 3.5 ton (average weight of a female) orca doing a speed of 50 km/hr.