Because your cs is ok. Because it was a blowout. Because kills stop mattering at roughly 5-6 kda. Also how many wards did you place. How much turret damage did you do. What was your damage dealt to champs.
But mostly because it was a blowout. The system realise your team outclassed the opposing team and graded accordingly.
Because other people have done better
cs, dmg to champ, objectives
stop chasing kills and play the game moron
It's about the kills. It's about CS, game time, kda, and objectives. I've gotten recently S+ on Anivia and i was 20/6/14. I lost the game but still got the S+...
Every single score is factored into your Ranking, and ranking is based on other players playing your champ in your role.
I've played 2/2/30 Janna support games and gotten only and A. My warding is sufficient for those games I'd like to say, but other players manage to ward and clear more wards as Janna. Other players manage to die a time or two less. Overall good, just not sufficient for the ranking against other Janna players.
Depending on your role, different things are supposed to be weighted more heavily.
Marksman needs heavy CS, at least 10 per minute. (Though you should shoot higher.) At 30 minutes into the game, 132 CS is not sufficient according to how this is usually weighted. Objectives taken is the next point of focus. How many structures did you participate in killing? Kills are 3rd on the list. You can objectively say that 19/0 is pretty good, but this can mean you probably should have won faster than 30 minutes. (Easier said than done to be fair.) Though looking, at 24 kills participated in, this means you didn't participate in 15 other kills.
It's never as easy as saying "I did good, why didn't I get a good score." It's how Riot weights productive behavior in gameplay itself. The more overall productive you are, the better your score. Not just "I killed a lot of enemies", but gold output, target takedowns, objective oriented targeting.
because you play a stupid champion?
because you're a noob who thinks kills are the only thing that matters?
it would be a miracle if you made it to bronze 4 with that op.gg. learn how the game actually works, and what sort of things it takes to win consistently and you might improve. if you're lucky.