Ah, Rare Candy Treatment makes me laugh.
Because Game Freak can be evil bastards sometimes.
...The literal answer is that they needed to make Marowak stand out from the other Ground types...though they could have made Marowak a Rock type and made Bonemerang a Rock move, but that is just me...
Better idea: Marowak stays Ground, but Bonemerang is Rock. The choice between EQ and Bonemerang is divisive enough as it is, but I think everyone would pick a Rock-type Bonemerang over Stone Edge any day. You lose the extra crit chance, but gain more accuracy as well as sub-breaking abilities.
The concept of a bone boomerang doesn't really have anything to do with the ground, or rocks. If anything it should be flying lol.
The only possible explanation I could come up with is that the bonemerang is meant to stay low to the ground, so flying types can just fly over it. Because, you know, boomerangs aren't supposed to go high in the air or anything.Nintendo logic... https://i.chzbgr.com/maxW500/6176208896/h2A7B83D9/
Nintendo logic. Bonemerang is Ground-type because Marowak is Ground-type. Because, you know, Marowak absolutely needs STAB on its signature move, of course.(irony) What would you expect him to use, EQ?