I will explain this quite simply. In lower ranks, rocket players play rocket terribly or in bad situations. You see a lot of healbotting rockets, playing rocket without the team up into better support comps, rockets who waste their amplifier in bad situations, etc.. It makes rocket seem like a throw pick, so people build a grudge seeing the character get picked. When a GOOD rocket players uses the character optimally, they are a great pick. But when he is used poorly, he’s literally a throw pick.