Riot looks to K/D (Kill/Death Ratio) instead of KDA (Kill/Death/Assist ratio ) on junglers if I'm correct. If you have so many assist it means your team got the kills by their own effort and you were a minor factor in the kill. Like you ult 4/5 man and your team does all the work after that. And you have not placed a lot of wards/pinks nor did you destroy any if, I'm correct? S-/S/S+ needs an amazing performance because they look to the top x%. You also didn't have a good dragon control. The enemy got 3 dragons and you only 1. Not sure if Riot will look to those things...
Anyway, I would have hoped you will get S- tho. But it seems like Riot doesn't like you. Rip you.
Hope you will get S-/S/S+ next time ^^!
WP/GG