It's mainly because Bonemarang is unaffected by Grassy Terrain while Earthquake has its damage cut in half.
Considering Tapu Bulu is probably the least used out of the the guardians (Weakness to steel that his secondary typing doesn't cover and a quad weakness to poison, an offensive STAB that is literally ignored by Scizor, Celesteela, and Heatran although Heatran fears a Superpower, a terrain that sometimes helps your opponent more than it helps you, and vulnerability to Toxic Spikes without access to Defog) this reasoning is somewhat weak, especially since by virtue of typing Alolawak is pretty much a hard counter to Tapu Bulu anyways.




