The question is why didn’t the Emperor teach Vader force lightning if he taught Dooku. My original answer was because the prequels suck…in other words, he shouldn’t have taught Dooku in the first place. The reasons for Vader not using force lightning should be based on the OT, not the PT. And based on the OT, there’s Vader and The Emperor and that’s it.
I always just assumed he wasn’t as powerful as the Emperor. And I see no reason to abandon that theory.