>>46652045Golem's main role in OU is countering Zapdos, which Rhydon simply does better, especially since it hits harder while the difference in bulk is pretty negligible. For that reason, it fell to UU.
This post:
>>46652798 explains why it basically can't perform at all in UU, so it fell again. Also, it's bad against the common psychics due to its poor sp. def.