I realized all china Pokemon youtuber started to use High Horsepower more than Earthquake on Rillaboom , but i'm pretty confused about it due to the fact that High Horsepower has 5 less Power Acc and with no additional effect, could anyone answer me about that? Even Nebby judge me cuz of this!
In Grassy Terrain, the power of Earthquake, Bulldoze, and Magnitude(which is unusable in gen 8) is halved.
Since Rillaboom automatically activates Grassy Terrain when entering the field due to its hidden ability Grassy Surge, everyone tends to use High Horsepower since Earthquake's base power drops to 50 in grassy terrain, while High Horsepower stays at 95.
Hope this helped you!
Because when Grassy terrain is active, Earthquake's power is halved, while high horsepower's power is not.