because you aren't averaging more wins than losses.
that player will be on your team for 1 match. if you're failing to win more matches than you're losing then you should be demoted.
consider your actions during the champ select, the loading screen, the initial spawning sequence, the invasion period, laning phase, drake spawns, herald spawns, skuttler spawns, rune spawns, baron spawns, dragon spawns, first turret, first blood, and the procurement of bounties and distribution of gold.
think about all of that.
think about your build.
consider your efforts and reflect on what you could have done to improve your odds of winning.
did you make a good ban?
did you check to see if your team was on a losing streak before starting the match?
did you make sure to play a balanced team?
were you autofilled?
did you defend the invasion?
did you ward?
did you assist by roaming against jungler ganks?
did you last hit effectively?
did you win your lane?
did you help others win their lanes?
are you playing a champion that can benefit the team early, mid, or late game?
and most importantly
DID YOU COMMUNICATE EFFECTIVELY?
the primary reason most players lose matches is due to the lack of communication in low level play. 1 person knows what they want to do and assume that everyone else knows or will follow along without hesitation, but that doesn't work.
so since you're being demoted then it's probably very likely that something on this list or something that should be on this list is what's causing you to fall in ranks.
yes people are bad at the game and tend to die a lot and make the game amazingly hard for everyone, but also there are 5 players on the enemy team just as there are 5 players on the friendly team.
if we're talking traditional lanes rengar isn't a top and riven isn't a jungle. so that alone tells me something is off about this situation.
if riven is dying to the enemy jungler then it's the allied jungler's fault for not contesting the enemy jungler. just as if support goes mid, but doesn't help bottom if the enemy jungler goes top and focuses 2v1ing top then that's a call for the allied jungler to follow and provide support.
if riven is jungling then that's a problem in and of itself, but he least the team could do is support her as she tries to take camps.
feed a 
5 kills by the 5 minute mark who then steam rolls the entire team to a point of obsoletion and finishes 20/6 in a 25 minute game? Why am I being punished because one shitty player who clearly doesn't belong in this elo literally ruins the game for the whole team? It blows my mind this is how your games rank system works. I have the power to ruin everyone's mmr that I come in contact with if I feel like it because one player can't singlehandedly win a game, but one player can singlehandedly lose it.
