It's also tied to your performance compared to other people playing the same champ in the same role (or presumably the same map, since ARAM has no roles). I imagine the top tier players of Kog'Maw in ARAM have a higher KP% and fewer deaths. As much as I understand the struggle to get an S or S+, I also realize that there's a reason why it's so hard to get them: cause there are people who are still notably BETTER at that champ than me, and if I want to be in that elite tier, I have to "git gud".
This is sometimes why you'll notice it's way easier to get high ranks on new champs - cause a large chunk of the the player base hasn't figured out fully how to play them yet. If you're a really solid ADC and took to Xayah like ring to a bell when she came out, it wouldn't be that hard to see S ranks pretty regularly until she became more mainstream.