>>38499944A few reasons:
One, Burn is one of the most devastating status conditions to be inflicted with since it halves the attack of the mon' and deals chip damage each turn. Any physical attacker wants nothing to do with it, and the move itself can do decent damage when combined with Scald's burn chance.
Speaking of which: the second reason would be it having an extremely high burn rate, at 30%, which triggers quite frequently. This means it has a very high probability of screwing over anything that relies on physical attacks to fight back, and can both burn and damage something when they switch in. Adding to that, most fire type moves have a very paltry 10% chance to burn, which while balanced, makes it seem pretty illogical in comparison that boiling water will burn you more than being set on fire by burning chemicals or something. The only fire type attacks with a similar chance are Lava Plume (which is literally fucking lava) and Sacred Fire, which has a gigantic 50% chance to burn, and is only known by two johto legendaries.
Three, Scald has an EXTREMELY large distribution. Almost every water type that's not water/ice can learn it, along with a paltry handful of non-water types, meaning that just about everything that has it gets stab off it. Combine this with the fact that most users of Scald are Bulky Waters, which almost guaranties they'll get to use Scald multiple times, (two scalds have a 51% chance to burn, etc), and can then easily heal off damage. Examples include Suicune, Quagsire, Toxapex, Slowbro/king, and more.
Four, it outclasses most other water type moves just by existing due to that massive 30% chance to burn outclasses the tiny increases in power. The only move I see used over Scald in singles is Hydro Pump since those 30 points added make a pretty significant difference, but most things will prefer the ability to cripple switch-ins. Plus, the only type immune to Burn, Fire, is weak to scald, meaning they can't absorb them for you.
