Why is this a thing?

Why do I get placed in games with constant trolls. I understand people have bad games, but come on. How is it I get in six straight games where someone feeds the enemy laner 7+ kills and then I get ganked by said enemy laner and it snowballs the rest of the game. I don't know why I go from being in a series to get out of bronze and into silver. To being demoted after 6 straight games of me not doing bad. (I play support, but generally in the majority of those games I had more assist then deaths.)

I'm just saying this is complete BS for me to be demoted for my teammates feeding. If I'm playing against an evenly matched opponent then it is impossible for my team to stop the snowball. There needs to be punishments for these players who feed a lot more than average.

Hate me all you want, this is complete BS to be demoted after losing a game where my teammates put me in losing situations.