This is why:
First off, Huge Power doubles the actual attack stat and not the base, so Azumarill hits harder than base 100 attack. Fun fact: its attack stat after Huge Power is equivalent to non-Primal Groudon's attack stat, which sits at 438. Groudon has base 150 attack. Let that sink in for a moment
Water-Fairy is an amazing typing, Fairy being able to cover Dragon's resistance to Water, and Water making it neutral against Steel types
It has access to Belly Drum, which makes it even more of a nuke, and it also has access to a stab aqua jet, making it perhaps one of the most easy sweepers in the OU metagame.
Hope this helped you :D