First off if anyone tells you it is because you are a support main they are wrong! I have been a consistent support player since season 4 and I have made it all the way down to Platinum II at best and I continue to play and am currently in gold as a support main. Second, you are bronze I and unfortunately in that low bronze/high silver are players who don't necessarily focus on objectives or make correct decisions. (Instead of going for baron they may chase down the mid laner across the map) I have seen it all when I played in those divisions and once I got out of them, once you start getting to mid silver you will start to see better players.
You seem to have pretty good stats and a good win rate as support so I just say keep doing what you are doing. Be positive, be smart, and focus on objectives rather than kills. Sometimes you can have the best games but without your team you can't win and that is just the way it is. Everyone has those games and everyone has huge win streaks and huge loss streaks. Keep playing to improve and you will be successful!