It is a confirmed fact that MMR is not a factor in grading (for any role).
Higher elo players tends to have good habits/skills that lead to them getting S's more often than low elo players, but low elo players still manage them sometimes, presumably because the fact that they themselves are bad is sortof canceled out by the fact their opponents are also bad.
If you're a diamond playing against bronzies, you'll probably end up with near perfect cs and a godlike kda so sure an S would be easier, but if you're a bronze playing against bronzies, you'll not get anything particularly special for kills/cs since you'll be making mistakes and not capitilizing on their mistakes, plus you'll simply miss lots of cs even if the other laner isn't actively denying you. It's certainly easier to get an S when you're stomping people at a lower elo than you, but usually people end up roughly where they belong so things become more 'fair'.
In any case, KDA and cs aren't the only factors. Kill participation, wards placed, wards cleared, objective participation, etc are all factors as well, and theres probably more. You're not graded relative to your teammates, you're graded relative to all other players (regardless of elo) using the same champion in the same role, and so if you're not getting an S, you're simply doing it worse than others have managed.
I'm a mid plat support main, and I pretty much always get at least an S- when playing janna, and I usually play champs in the support role when it's time for me to earn my weekly S rank for a chest. As a support main, I tend to find support is the easiest role to get S in, simply because I'm good at the role. People who main a non-support role will likely find they're best at getting an S rank in whatever role they do main, and I suspect a lot of people who main non-sup (and perhaps not jg) aren't used to the warding/sweeping habits required to be a good sup.