It's cause of the sh**ty matchmaking system, it's set up to make you lose instead of actually pairing you up with people of relevant skill in all of your matches. You're gold? Great now you have to carry silver, bronze and unranked. Congrats, you made it to plat, now you have to carry gold! Ohohoho, you actually made it to diamond, sweet now we're gonna make you carry plat.

Instead of consistent quality games that match you with people around your skill level, the matchmaking makes you carry weaker players in order to rank up. The thing is, if you're playing support, you're basically F'd. Also, if you win a game, the next game they're gonna match you with atleast one weaker player. Unfortunately in RANKED CC, every single player matters, if one person doesn't pull their weight, things can go sideways real fast. They really need to overhaul matchmaking to match you with people of your skill range consistently, regardless of if you lost the previous game or won the previous game.