What is this ranking system?

So I'm currently doing my ranked placements. I have 8 games played of which 3 were wins and 5 losses. In the last game I just played the other team's toplaner was Ashe, ours was Jax. After dying twice in the first minutes of the game Jax quit. My support was going in when I (the adc) was low on mana and health forgetting 1. the other team had a morgana support and that their adc would kite her a** off and 2. we shouldn't all-in when we are already behind. The game ended after 18 agonizing minutes of me telling my team to group, not go into the jungle by themselves, defend the base, not 1v3(my support), etc. and I go to check my mmr and it dropped drastically and it says that I would end up in B2. I have never been in bronze before and I'm certainly not trying to end there. I've ever been to s4 and in the preseason to s2. My question is this: Why does my freaking mmr drop/rise because of a loss/win? Shouldn't the ranked system place people based on their performance and abilities rather than their ability of single-handedly win a game? Because I find it bs to put people in lower ranks because of their teammates not playing well.