It's a lot of reasons why you didn't get higher than a S-. The grade bases your score based off the average of what people in your region get with that champion, in that role. Since you are playing
the average grade is generally higher since a lot of people play that champion, so usually the more popular of a champion/role you are playing, the harder it usually is (but not always).
The grade is based off a lot of things and not just your KDA. It grades also on your CS, and I think even towers destroyed, and wards placed/destroyed to. Your CS wasn't very high, which probably brought your score down quite a bit, though I can't see anything about wards/towers since you did not show or tell that information.
Also you are comparing your scores to the support. Remember that supports aren't usually getting high kills, or CS. So for this
to have the same amount of kills as the Mid and ADC probably raised it's score up quite a bit, though it's not all that matters for scores...Because I've gotten S ranks with
with like 2 kills, 0 deaths and under 10 CS because my wards placed and assists were really high.
Also I think damage dealt is another factor with it, and since the Morgana was full AP, I could guess she had some really high amount of damage dealt to champions. And as for Gold earned...Morgana was also barely under you and had more gold than every else but you.
So basically for a tldr: Your damage dealt might of been low, you probably didn't clear to many wards or placed them and your cs wasn't above average but your KDA was so high that it boosted you up quite high on that alone (most likely). If you had more CS, then you might of gotten an S or S+.
Not trying to sound like you did bad, because obviously you did very well but when comparing every score to the average Nidalee Jungle wasn't as high as the Morgana supports score to the average Morgana support.