It's not that it's hard to get S on support but...
It's lame that you have to play suboptimally to get it.
If you want S on support, just just need to try to KS as much as possible. (actually, that's how to do it with any role, but for most roles it's not as bad to do this.)
Yes, KDA isn't the only factor in champion grading, but it is a very big one.
Champion kills should NOT be weighed as strongly for supports / tank champions as it is on other champions. A 7/5/10 thresh usually played worse than a 0/5/30 thresh (with the team scores). Yet, the support with higher kills than deaths usually gets a better score.
Sincerely, semi-support main.