>>27076821Same reason they made Nidoran F and Nidoran M, or Volbeat and Illumise, or Beautifly and Dustox. Just two slightly different versions of the same Pokémon because they wanted to do something a bit different.
Also, Slowking has been used at VGC Worlds three times, and Slowbro has never been used, so Slowking is obviously the best.