Why does smogon recommend crunch over waterfall for Gyarados in it's Mega DD set?

Just out of curiosity, why is this? When you look at Gyarados' smogon page, the mega dd set has crunch, earthquake, dragon dance and substitute with ice fang or waterfall as an option to replace sub. So what’s the reason for this? I mean, when you use Mega Gyarados, timing is one of the things you need to think about and before mega evolving, crunch doesn’t have stab. So, shouldn’t this mean that waterfall would be the preferred stab?

Pleaselog inorregisterto add a comment.

According to thetype coverage checker,

Dark/Ground:0 No Effect26 Not Very Effective608 Normal Effectiveness400 Super Effective

whereas

Water/Ground:1 No Effect117 Not Very Effective582 Normal Effectiveness334 Super Effective

and

Water/Dark:0 No Effect34 Not Very Effective680 Normal Effectiveness320 Super Effective

All of this is not counting the Water Immunity some Pokemon have due to their abilities (Water Absorb, Storm Drain), so the ratio is actually skewed further in Dark/Ground’s favour. So in general, simply by the numbers, you’re a lot more likely to hit more Pokemon for more damage with Crunch/Earthquake than Crunch/Waterfall or Waterfall/Earthquake.

Of course, there are some exceptions to this, and the most popular sets will be the ones that can counter the biggest threats, i.e. a Crunch/Waterfall set wouldn’t be as effective in a meta where Greninja is super popular, whereas Earthquake wouldn’t be favoured in a Grassy Terrain meta. It’s a matter of what sort of opposition you might have to deal with, but whoever made the set probably took into consideration data from a lot of battles which suggested that Crunch/Earthquake would over many battles, be more effective than the other sets.

The Smogon page already answers your question.

Crunch is a strong primary STAB move that gives Gyarados good neutral coverage and hits Psychic-types such as Mew, Latios, and Slowbro super effectively.

Waterfall is an option that hits Ground-types like Landorus-T for super effective damage and hits certain Dark-resistant Pokemon like Clefable for neutral damage.

sourceSo crunch is super effective against more threats, and most Pokemon with water weaknesses don’t resist dark.