>>32927594That, and most Electric-types are too frail to make use of Paralysis' effects, so Thunderbolt is always favored over Discharge outside of doubles. Fire-type is also not a very good typing to have, so Lava Plume isn't a big deal either.
Scald's issue, compared to those two attacks, is how widespread it is (literally almost every fucking water-type can learn it barring a few extreme examples like Magikarp), and the fact that they can all make use of it. Most Water-types are bulky, and burning your opponent at a 30%(!) chance with a decently powered move makes them even harder to kill. And unlike the previously two mentioned moves, you can't switch in a Fire-type to prevent burns because you'll get hit with SE damage.