The removal of Earthquake in PLA really shows how entrenched it has become in the games. I don't mean just competitively, but also move design-wise. It has been the go-to physical Ground-type move since Gen I, and there have been few moves stronger than it since then, or close to it in power. A lot of them have been signature moves, or moves made for pokémon with specific features. High Horsepower and Drill Run were made foremost for pokémon with burly bodies and pokémon with horns, respectively, while Earthquake was made as a generic Ground-type move that pretty much every Ground-type can learn.
This is why the removal of Earthquake in this game becomes a problem, since High Horsepower can't really be a full substitute for it. Some Ground-type pokémon simply don't fit with it, so they got left out from having a strong Ground-type STAB.
I think Headlong Rush could've been a good substitute for Earthquake, since it is more generic in design and would seem to fit more pokémon flavour-wise.