Maybe because a plane can glide, but a helicopter can't
Well, that'll be news to most of the helo pilots here though I can't for the life of me imagine why or how lightning might knock out engines. Does this happen in flight sim on your pc then?
Lightning passing between the rotor system and the structure of the helo must necessarily pass through the gearbox. This can result in a momentary spot weld occurring between meshing teeth in the gearbox which is of course instantly pulled apart as the gears rotate thus leaving damage to the meshing faces of the teeth. Equals strip and rebuild gearbox - expensive.
Otherwise plastic rotor blades (main or tail) can be physically damaged by lightning thus potentially causing an immediate hazard to flight in a way that would not happen if the flying surfaces of a f/w were damaged.