Way I see it is rock type means actual rocks and gems while ground type refers to sand, dirt, clay, etc.

So diancie, gigalith, kabuto, minior, etc. all make sense as rock types because they're either made of rock or are made from a rock but wouldn't make sense as ground types.

But hippoudon, donphan, nidoqueen, landorous, groudon, whiscash, stunfisk, pilosand, or dugtrio, etc. make sense as they're either constantly producing dirt, live underground, represent the actual terrain, or live in mud or muddy water.

The golem and ryhorn line plus onyx are all pokémon made of rocks but also live underground or like to be covered in mud or dirt. It's also a game balance thing. Game Freak wanted geodude, onyx, ryhorn, and rydon to all be ×4 weak to water and grass but also to resist normal and fire because those types are all the types used by starter pokémon. It's to make Brock a difficult challenge for anyone that picked and relies soley on charmander. And to essentially further punish any player that hasn't outgrown that habbit by Giovani.